Every physicist (and even a whole lot of non-physicists) knows the Big Bang theory isn't perfect. The closer you get to tiny fractions of second after time=zero, the more problems get thrown up. However based on our current state of scientific observations and knowledge, it is the closest thing we have to explaining how the universe sprung into existence. If you want to say "God created the Big Bang", scientists generally wouldn't care, except to say "that's fine, but invoking a God to explain something is not in our job description - we look for real-world explanations".
The assumption comes from the fact that the vast majority of religious people believe that their god has a noticeable influence over things that happen, answers their prayers, or when he doesn't answer them it's for a specific reason which they don't understand but he must know what he's doing anyway, protects them and keeps them safe (except when he doesn't, then again it must be for some reason they don't understand), etc, etc.
If you believe that a God created the universe then left it alone and takes a hands-off approach to everything, then you are very much the exception rather than the rule! Most Christians believe God meddles in pretty much everything, every day. He's pretty bloody busy. Heck my parents even suggested to me once that the reason I got into a well paid professional flying job was because they prayed for me (yeah thanks a lot mum & dad - way to compliment your son - couldn't have been anything to do with years of friggin hard work could it?).