In large part it's a symbol for fear of a religion, Islam. The clothing is alien to most people who read the Washington Post, a symbol to them of a religion that's alien to them. It's an excellent representation of fear of the unknown (which is exactly what the terrorists, lacking the resources to achieve their goals any other way, want to create: Terror, fear, to drive their targets to create large-scale conflicts that the terrorists cannot create themselves and otherwise nobody in any religion would want.)
Social norms and rules exist everywhere for clothing, especially for women. Women in the west must cover their breasts, an absurdity in some other cultures (I vaguely recall an interview with women in one non-western culture, who laughed and asked, 'are men there babies? they are so obsessed with breasts?'). In the workplace, at houses of worship, and other serious places women must cover more of their bodies - bikinis generally are frowned upon! Other countries are more strict: In India, the strong custom, if not rule, is for women to cover themselves from the neck to the wrists to ankles (except in movies and ads, which are like the West). Other religions have rules too: Many orthodox Jewish sects required women to be fully covered, including something on their heads. And in the West, every woman's clothing is apparently fair game for everyone else's opinions. (And there are rules for men to, but far less attention to them.)
Note how everyone likes to tell women what they can wear.
EDIT: Some minor edits