Possibly even a better indicator then asking for religious preference as only the true believers would order a specific meal type.
Right up until some true believer realises that if they're blowing up the plane they can skip lunch, and so orders the beer battered onion rings & pulled pork, to throw authorities off the trail.
Given the purpose of any proxy would likely be identifying muslims, and they practice fasting every year, I'm pretty sure they could manage a flight without eating, thus making food a very poor proxy for that religion. So while I don't have any good idea why they want to collect that info, other than "because we can", I don't buy the religious proxy argument.