In the case of philosophy of religion I think what's happening is a selection effect: people who believe in theist religion are disproportionately likely to think it worthwhile to study philosophy of religion, i.e. the theism predates their expertise in the philosophy of religion, and isn't a result of it.

I'll give you a slightly different spin on the bias. More evolutionary bias than selection bias.

People who assert that a field is worthwhile are more likely to be successful in that field.