because when you say you know whether there is a god or not your really telling me your full of tacos.
No. They are saying they are sure no God exists because they have seen no evidence to the contrary. Usually it seems, in everything but this, that is a safe way to come to conclusions. Lack of evidence = lack of belief.
There is no objective reason to bring a God into the equation at all if you can in any way look at the universe objectively, and stop thinking about humans as the be all and end all of everything.