many theists don't assign a probability of exactly 1 to God existing either.
Believers do not assign probabilities in the matters of faith.
Well, I guess there are lots of non-Bayesians among non-believers too. And I've heard quite a few believers who, while they were likely not familiar with the concept of probability, made a distinction between belief and knowledge (IIRC, a priest saying “the devil believes in God more strongly than any man, including the Pope: he knows God exists because he has seen him, unlike the Pope who hasn't and so might still have some doubt”.)
http://bigthink.com/think-tank/neil-degrasse-tyson-atheist-or-agnostic
Apparently Dawkins and Tyson give a non-zero probability to "God". Which is pretty much what is expected of a rational person. And of course it will be used by theists to say "They aren't really sure!"