I'm trying to evaluate this situation by imagining that a Church (Synagogue, etc.) put an analogous advertisement on buses....I can imagine disagreeing with the message, perhaps even chuckling about it, but I don't see myself feeling it is "low" of them to put the message out in public.
That is, any problem I would have would be with the message itself, but not the act of putting the message out in public by whatever free, legal means. If people have something to say, and they think it is important, I encourage them to say it. Then, let's talk about the merits of what is being said, not the merits of saying it.