^Because typically the Muslim girl doesn't have a choice in the matter, and stands a very real chance of being maimed or killed should she exhibit thoughts or actions that suggest otherwise. :-/ If you could be born a nun (as opposed to joining as a deliberate action taken by a willing participant), and the Catholic Church beheaded (or set fire to, or doused in acid, etc) nuns who attempted to leave the faith or even indicated nothing less than total submission to the will of the Church/their male family members, then maybe there might have been a relevant point somewhere in there.
For the record, I acknowledge that there are always exceptions, and I find the Catholic Church to be horrendous in it's own fashion so I'm explicitly NOT taking their side either.