Because most people follow a neutered form of religion that isn't an acute or extreme threat to democracy and freedom. It is a diffuse threat to some freedoms for sure. But it doesn't compare to actual ISIS or fascist propaganda. There is no way to categorically separate these things. It's about matters of degree and pragmatically deciding to draw a line somewhere, instead of being ideologically pure and pretending that the best thing to do is draw no line.

You do know we have documented proof that broadcasting hate speech tends to cause people get violent, right? Should we allow free radio of a thousand hills [0] to broadcast?

[0] https://en.wikipedia.org/wiki/Radio_Télévision_Libre_des_Mil...