Some post religious topics solely for the purpose of stirring up conflict (That doesn't necessarily encompass trolling), but others are actually interested in discussing religion in an orderly and respectful fashion. There's no reason to stop people from discussing those sorts of topics if they're all being polite about it. But too often people seem to misinterpret "Well, I think this is wrong, and here's why" statements as rude or disrespectful. They aren't.

For attention and to antagonize. None seem to honestly be made for true discussion. They most all seem geared intentionally at making one side or the other angry, to make fun of, or to berate. None of which is productive or worth anyone's time. That said, no one side is going to sway the other into thinking that their views are right. Not here, and not this way.

it's a surefire way to get lots of replies.

Because they're not banned and are instant hot topics.

Probably a combination of both, but more likely it is mainly the latter and it is an easy way to "troll" people.