To publicly make a huge deal about atheism is to spit in the face of all of those people who enjoy religion without any tangible benefit to yourself.
But that's the thing, what I love is how non-huge-of-a-deal the buses are. Here's what they say:

OK, so the first part says, "there's probably no god".
(And I also love that they managed to avoid the capitalization issue!)I don't see how that's terribly offensive. The word "probably" is well,
probably there mainly so that they don't break any laws about false or unprovable advertising, but I really like the way it's phrased. It sums up exactly how I feel as an atheist - the existence of a god is very unlikely considering all the evidence we currently have, but who can say for sure? I'll never understand atheists who claim to KNOW that there is no god. To me that's as stupid as blindly believing in a god just because a bunch of other people do. In any case, if people of faith are as secure in their convictions as they claim to be, then how are they being hurt by the mere suggestion that they're wrong? I thought faith was supposed to set people at ease. If they're bothered by an ad on a bus, doesn't that really say more about how insecure they truly are?
Second part says, "now stop worrying and enjoy your life". This I'm not as keen on, simply because I feel it could be softened a little by using "so" instead of "now" and by using an exclamation mark at the end.
"so stop worrying and enjoy your life!"
...sounds friendlier and like less of an order to me than "now stop worrying and enjoy your life." Starting an imperative sentence with the word "now" implies that not only is this an order, but this is an order that needs to be followed immediately.
All that said, requesting that people stop worrying and enjoy their lives hardly seems very offensive.
Are you going to CONVERT them to Atheism? That's the dumbest shit I've ever heard. Who the fuck would waste their time evangelizing about non-participation?
You're missing a major point here, man: compassion.
Why do churches have signs with various messages for all to read? Why do churches and church-goers actively try to recruit others to join them? It's because they honestly believe that they can help these poor non-believers. I'm not talking about how the Catholic church owns half the planet, or anything like that. Sure, at that level it's all corrupt as fuck. I mean regular people, the kind of people we all know, the kind of people we're all related to. The vast majority of religious people are good folks who happen to believe in something I don't. And the same vast majority probably feels bad for me since I'm on my way to hell or whatever. The compassion they feel towards us non-believers can end up being super annoying, but that doesn't mean that their hearts aren't in the right place.
You know how there are many cases of people not being particularly religious, then suddenly discovering the faith that fits them, that makes it all work for them? Well think of just how many people are trapped the other way around. I know a fuckload of people who were brought up religious, went to church all their life, never questioned any of it, and were miserable. Then they discovered that they weren't alone, that it's OK to not believe, that there are others who think the same way and don't feel any shame about not believing. And the one word that I've heard from people like this over and over is "liberating". It's kinda like coming out of the closet, although I'm not sure that being an atheist isn't less socially acceptable than being gay.