Look, what's basically being suggested here is that atheists shouldn't speak about atheism, because it might upset the religious.
In doing so, you've indicated that the religious have more of a right to freedom of expression than I have.
You may not think that's what you're saying, but you really are.
It's nice that the busses are in a small island nation that doesn't have quite the focus on religion that my country does. In my country, I doubt seriously that message would ever be seen on public transit. The howls of protest would just be the background music for lost revenue, because people would stop riding those busses, would stage strikes, would stage protests.
Or did people forget that this is the nation in which Harry Potter book-burnings were staged?
Freedom of speech doesn't protect approved or acceptable speech. Approved speech has no need of protection. It's the stuff people would howl about, would silence, that needs protection, so that it can be said. That's freedom.
If you just go along, coddling the insecure and superstitious, never saying anything that might "rile them" intentionally or not, ignorance wins. Suppression of free speech wins.
I'm going to ask again, because I haven't seen an answer:
Is there anything that can be said by atheists in support of atheism, in a public venue such as the side of a bus, that wouldn't offend some religious, somewhere?
If the answer is "No, statements in support of atheism would probably offend at least some of the religious," then this "don't rile them up on purpose" argument is purest bunk! Our mere utterances about atheism rile them up, don't you see? To say that you cannot speak because people might be offended goes to the very heart of our right to free speech. You're actually trying to tell me that someone's feelings matter more than freedom of expression; that I should be silent, but they are free to speak.
The other thing to consider is that here in the U.S., taxes fund busses. And if U.S. tax dollars (yes, local and federal) help pay for having mass transit, then the religious shouldn't ever be allowed to advertise churches or say anything of a religious nature in a bus advert. Religious TV programming shouldn't be advertised on a bus; religious figures shouldn't have their faces plastered there; religious organizations shouldn't even sponsor an innocuous "have a nice day--brought to you by 4th Street Baptist church" message.
But they do.
And no one making the "don't poke sticks at bears" arguments even pauses to think about those of us who might be "riled up" by having religion shoved into our faces, sponsored by our own government on all levels, from city to state to federal. Maybe you don't mind riding to work each day with Pat Robertson's crazy, lying face plastered right under your elbow, but I'd resent it in the extreme, especially if I knew my counter-opinion can't be similarly expressed because someone else might be offended.
Complacency never changed anything. Women and minorities didn't get the right to vote by being nice and quiet and trying not to "rile" anyone. Slavery didn't end because the silence about it was so deafening.
But you want us to remain silent, say nothing, don't upset anyone, because it isn't nice???
The fact that people are arguing about whether this is something that ought to be said indicates to me the answer is not only YES, but YES, IT MUST.