This is America. Obviously, her parents weren’t all that devout - like hordes of other nominal muslims. And in America you can choose your religion. I know two Iranians who converted to christianity. Happens all the time.
In Islam it doesn't say "devout" or "not devout" as to whether the children are Muslim. If the family is Muslim (which they say they are) then the kids are Muslim... and that's "Islam" -- not "America" ... doncha know.
Furthernore, yes people can "actually" leave Islam (as they do so all the time; many examples of converting to Christianity) -- but they are subject to being killed under the religious imperative of Islam, and anyone in Islam can kill them and it's considered commendable and "okay" for that adherent of Islam to do so. That's why those who have converted and are also prominent and well-known people and have declared that, are very careful about their security. They know that they can be killed by some Muslim at any time.
HOWEVER, in this case, it's also clear that this current "Miss USA" is a Muslim, because even the "media" is "declaring" -- "First Muslim Miss USA" -- and since she has not declared her abandonment of Islam (as others have, in declaring themselves to be "Christian" for example) -- then she's a Muslim, defacto -- and by her "word" (or rather her absence of declaring for another faith).
I don’t give a sh*t what islam says. Obviously, if one rejects islam, then they don’t care either.