Their antipathy isn’t towards God. It is directed at His followers. News for the atheists, God loves you too. Unrequited love. An age old story.
I don't have antipathy towards anyone who is a believer, unless they go out of their way to tell me that I will suffer eternal damnation and torture because I don't agree with them, and that I can't be moral without following their particular god.
Why get mad at unbelievers who use popular media like bus billboards to make a point, much like many religious groups do?
With regards to the billboards themselves, I don't see why a Rastafarian hippie needs to be the symbol for atheists. Couldn't they have used someone less goofy?