Atheist groups have different tactics. Some want to be offensive precisely so they can then make the argument that NO religious displays should be on public property. It's either all or none, many atheists would prefer none over all being allowed, so being offensive tips cities toward that direction. Yes, cities have decided to allow none so as not to be caught in the fight.
If you really think about the Christian message, that if you don't believe, you deserve hell... that's a whole lot more offensive than an atheist saying your religion is a myth.