Why are some offended by the very notion of atheism?

Just stumbled on this. It’s another instance of what is becoming a common occurrence. An atheist group puts up a very tame billboard, and everyone flips out. This one in particular reads:

Being a good person doesn’t require God.
Don’t believe in God? You’re NOT alone!

How anyone could be offended by that, I don’t know, but of course the local community is lobbying to get it taken down. Judge for yourself, but from reading the quotes in that article, it appears as though many protesters are offended by atheism itself, rather than any offensive message that the sign might have presented. Why is this?

Please, refrain from the “because they’re idiots” response and the like.