What you need to understand here is that wearing a hijab or niqab is often not completely "her choice". Therefore it's seem as some form of opression.

First of all,I'm not sure if Islam says is mandatory, but not all religious folks go with it side by side.

For rational people, is not a secret that women are below men in most religions, specially Islam. I personally think these clothes are another shenanigan the patriarchs that wrote the Quran, used to "protect" their women (one has several), from other men. Same with the celibacy thing.

So, even though women say they choose to wear it, IMHO, this is just the Stockholm syndrome. The reality is that women raised on Islam families suffer a lot of pressure, because in their irrational tradition, the family has to decide the live of the woman. If the woman reveals against this, there is one common consequence: Disinheritance, your family will not longer treat you as a member, etc. Sure, there are religious families more tolerable, but that's not often the case.

And there's an even worst consequence a woman can face if she reveals against family traditions: Honor killing.

So, having those consequences in mind when "choosing" to wear or not a hijab, is not choosing at all.