Well, I think many theists don't assign a probability of exactly 1 to God existing either. (I'd only call someone agnostic if they assigned odds against God existing “of order unity”.)
many theists don't assign a probability of exactly 1 to God existing either.
Believers do not assign probabilities in the matters of faith.
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!"