That may be so, they weren't living in an alternate universe though. One were there are quite literally creatures that will kill and eat you if you are anything less than ruthless.
I find this to be quite absurd, people will fight tooth and nail against aliens being in a DLC but actually 'want' something as ridiculous and contrived as 'religion' in an RPG/FPS...
Religion is part of human nature, and as far as I'm aware human nature is one thing that isn't different in Fallout's alternate universe. The fact that you or I might think it absurd is neither here nor there, churches still persist despite our disapproval.
Just to be clear, my position here is about versimillitude, it's not about ideology. It's not about shoving opinions down people's throats, but accurately representing patterns of human behaviour.
(Actually, I think some more stuff on post-apocalyptic religions and belief systems could be really interesting in a Fallout game. Look at religion and it's role in TES.)