Joan Smith: One thing is certain with sharia... men make the rules

It's no surprise that Lubna Hussein has been convicted by a Sudanese court. Sudan's President, Omar al-Bashir, came to power in an Islamist-backed coup, and holds the distinction of being the first head of state to be indicted by the International Criminal Court on charges of war crimes and crimes against humanity. This is not a state where anyone's human rights are respected, and women in particular are subject to arbitrary rulings of the local version of sharia law.

In Sudan, wearing trousers in public is now officially "indecent" for women; outside the hearing, Islamist demonstrators railed at Ms Hussein and her supporters and called them "prostitutes".

Yet in other Islamic states, she and her friends would have got into trouble if they hadn't worn trousers, risking the exposure of too much ankle. There is no consistency at all in the way sharia rules on women's dress are interpreted, except in this one respect: the authorities who decide what is and isn't permissible are men.

There are countries with large Muslim populations, such as Turkey, where some women cover their hair and others don't. Some clerics deny that covering is necessary at all, while others demand the wearing of the hijab, niqab or even the burqa. What's acceptable for Muslim women in Lebanon or Syria would not be tolerated in Saudi Arabia or Iran, demonstrating that the rules are patriarchal – designed to maintain male clerical authority – rather than doctrinal.

At one level, what's happened to Ms Hussein is preposterous, even deserving of that overworked adjective Kafkaesque. But her conviction matters a great deal, and not only because of what might happen to her if she continues to insist that she won't pay the fine.

If we believe in universal human rights, one of the most significant is the right of equal access to public space. If a woman isn't allowed to venture outdoors unless she takes special precautions that don't apply to men – whether that consists of covering the hair or face or being forbidden to wear trousers – then she is being denied a fundamental human right.

Lubna Hussein should be hailed around the world for challenging this sexist and capricious practice head-on.