"Also, as I understand it, the bus ads are not in fact "atheist ads".   The word 'probably' is used.   That makes it agnostic, not atheist."

No it doesn't. Atheists are realists. We understand that there cannot be proof of the non-existence of God. Even Dawkins thinks that. To say that God definitely doesn't exist would be like saying that the cosmic teapot definitely doesn't exist. You simply can't say it, even though the likelihood of the existence of either is vanishingly small. The atheist says that the upshot of this is that God and cosmic teapots can, in effect, be totally disregarded. Neither is a notion that can even remotely influence the way we live our lives (except for all that ubiquitous iconography rubbing off - believe it or not, we atheists are only human). We have better things to concern ourselves with. An agnostic is a completely different animal. They believe that there is a real possibility that God exists and that nothing currently known can prove him one way or the other. They are fence-sitters. Actually, there are probably very few of them. Most so-called agnostics are probably people who simply don't give a bugger one way or the other.   

"Agnosticism makes perfect sense.   Atheism makes no sense--as well as being the worst thing to ever happen to humanity."

Atheism was the only sense for millions of years until you believers, you Johnny-come-latelys, showed up! We did muddle through reasonably well even without you before that, remember. Better, probably.