Because these kids grew up in a social context where allahu akbar was treated basically the same as heil hitler (except in the parts of the country where they prefer that), and now they're publicly announcing they don't give a shit and the stigma no longer means anything to them.

Just realized I should have specified: the students I heard if from were quite white, so I read it less as as heartfelt religious sentiment and more as:

1:A heartfelt declaration of allegiance to the enemies of the empire, using the words we were brought up to believe were some kind of magic words that suddenly made you an enemy of the empire. In this context, a rejection of American exceptionalism and the racism that undergirds it.

2: A refutation of the status of the phrase allahu akbar in American pop culture as a punchline, something that denotes the character using it as unserious, undeserving of consideration, and marked for a death that will be played for laughs. An insistence on the common humanity of Muslim people in the face of decades of propaganda saying otherwise.

3: A way to freak out the squares, to rub in to the Mike Johnsons of the world that their most hysterical fears about the youth hating them are coming true.