You will not be surprised to learn that the net curtain is a British, or, specifically, English device, as it allows examination of the outside world without permitting introspection.
The net spread with the rising urban middle classes, who began to worry that their betters might sneer at their interiors and their inferiors might steal from them.
Originally made of lace, the introduction of light polyester nets increased opportunities for hidden observing, and the net assumed its role as a cipher for suburbia and its supposed narrowness and nosiness. Now it is rarely met without its default adjective, "twitching", emphasising that it's seen mostly as a means of prying rather than protecting.
Its more organised consequence, Neighbourhood Watch, hasn't attracted the same opprobrium. The upper classes prefer shutters. Continentals, who largely operate an open-curtain policy, are puzzled by all this obscurantism. But then they, palpably, are not British.