Nor can they accept that, in reality, Diana is a royal has-been, that no matter how she tries to dress up her role as an ambassador for Britain or as a Princess of People's Hearts, she can never be the real star of the monarchy again. She is the woman who would have been Queen and now must settle for King Mother. She is in the wings - and, really, that is no place for an icon to be.
But in America she does not have to settle for anything less than being the star of the show. This was her first trip since the divorce and she chose her destination wisely. She was feted at the White House, joked with Hillary Clinton, helped raise at least $1m for breast cancer research and was hailed as a serious campaigner on health issues. Of course, in America the latter is really not saying much. Kermit the Frog could be hailed as a serious campaigner, too, if he went to the right parties.
In Britain, by contrast, she herself is seen as a health issue. The woman who brought bulimia out of the refrigerator and on to the TV screen is seen even by kind onlookers as an empathy junkie hooked on other people's suffering. She lurks, disguised, in hospital corridors. She may have the best possible motives, but it cannot be denied that this really is rather strange behaviour.
Yet Nicholas Soames was wrong to brand her mentally ill after her Panorama interview - she is not - but Diana will have to accept (no doubt with the help of her celebrity therapist) that no number of secret mercy missions can make her former subjects see her as their Queen of Hearts.
The Americans are really not bothered by any of this. They know a star when they see one and they make the most of it. There may be something obscene about the lunacy surrounding Diana's visit but, then again, America is good at obscene lunacy (remember the star treatment given to Gerry Adams not too long ago).
According to the Sun's correspondent, "Princess of Washington DC" Caroline Graham, the charity ball was notable for such antics as top television interviewer Barbara "Baba" Walters paying $25,000 to reserve a place near Diana. Cringe-making, but at least it was for a good cause.
The British have a right to snort and sneer. After all, this is the woman who, with her philandering husband, has brought the monarchy to a point where it has to hold its very own millennium-type convention to figure out how to exert yet another dollop of damage control. BSE may be the biggest crisis of the day but the monarchy is running a very close second. Constitutional change never seems far from the headlines these days. The ideal of the Royal fairy-tale family is dead - it's just that no one can actually imagine what a post-Royal Britain would be like.
Americans have only the vaguest notions of any of this. "Oh, do you live in England?" says the store clerk in the middle of Iowa. "How is Di, anyway? Do you ever see her?"
Last month, on the West Coast, just about everyone wanted to talk about Diana, too. They tell me they can understand why she had to leave Charles. In fact, they can't understand why she didn't do it sooner. "Doesn't she look great?" they say. And, of course, one can only really tell the truth: yes, she does.
This lack of understanding is key to the way in which Diana is feted. America has never experienced anything like this really. Hillary and her role may be the subject of endless jokes and debate across the country, but what is unthinkable is that she would divorce Bill.
What would she have to gain? She would lose power, prestige and would have to go back to flying economy class. If she is not Hillary, she is just another Oprah guest. Hillary knows this. She and Bill Clinton may have married for love - but ambition was not far behind.
This became crystal clear during the Gennifer Flowers episode during the first presidential campaign. Hillary stood by her man. She looked at the future and saw that she could either be another woman with a sad story to tell or First Lady. She chose wisely.
It is interesting to think what would have happened if Diana had been clear-headed enough to see things this clearly. Being a princess is by no means the same as being First Lady, but the roles are comparable.
It may come as a surprise to the British that the Americans didn't think too much of Diana's great Panorama interview. It was considered a bit of a disappointment, which is understandable in a land where people regularly admit to killing their relatives on prime-time television. Americans may have thought it was too stage-managed and rather sad, if predictable, to see Diana playing the victim's role so incredibly well.
But Diana has now made it clear that she is no longer a victim. One guest at the White House ball said: "She was on tremendous form. She looked amazing but it was her new attitude that everyone noticed, she came across as a woman totally in control."
Another told the Sun: "She had the crowd eating out of her hand. We've seen a new Diana emerge tonight. She has shown she is a match for anyone. And no one should be left in any doubt that she's a major player on the world stage."
This is clearly how Diana sees herself, too. As she twirled with Colin Powell, the man who could have been president is said to have serenaded the woman who will not be queen with the song: "Heaven, I'm in heaven, and my heart beats so that I can hardly speak ..."
The tabloids say that Diana will not remain speechless for long. The woman who made Martin Bashir a household name is looking for a way round the gag order that is widely believed to have accompanied her divorce settlement.
If you doubt it, just think back to November 1995 and that wide-eyed performance. Afterwards, Helena Kennedy QC said: "I thought she was very engaging and very honest. If she were in a witness box I would find her credible. But what we're dealing with is a modern woman, a post-feminist woman of the Nineties who feels that she's got rights and that is what comes across most strongly. She is certainly nobody's fool."
People in Britain keep forgetting this. There is more to Diana than meets the heavily mascara-ed eye. Her conquest of America was all part of her own grand plan. This is one divorcee who is not the retiring kind. It must also be nice for her to be treated like a princess again. Having returned to Britain, however, the glass slippers must be stowed away, for now, at least, she is not allowed to wear them.
Hillary obviously likes Diana, and she went out of her way to show it this week. Anyone married to Bill Clinton understands the idea of "come- backs", and that is what this week was for Diana. But Hillary also knows that she is on solid ground: America may love a winner but it absolutely adores a loser who can bounce back and be a winner again.
The Politics of Redemption are very much at home in Washington DC, where Mayor Marion Barry has had his own brushes with life's underbelly. This is a country where one First Lady's name is best known because of her anti-drugs and alcohol clinic and where every celebrity worth their headlines has had one or two brushes with drugs, drink or disaster.
Diana is seen as a survivor. This week a new blockbuster has opened in America, The First Wives Club; it is a feel-good movie for divorcees, and sweet revenge is in the air.
Maybe that is what lurked behind the smile on Diana's face when Gloria Gaynor's anthem "I Will Survive" was played at the ball. (Strange music for a ball, don't you think?) "She giggled when the song came on. You could tell she was really tickled by it," said a fellow guest.
I suspect that is the understatement of the year.Reuse content