That's the thing about atheism, it doesn't guarantee anything. Being an atheist doesn't make me smarter, nicer or in any way better than I was before. People who are confrontational like that tend to be bitter or self-righteous, and it's probably not much of a change for them. Certainly it's off-putting to people just like the evangelist who knocks on your door first thing in the morning.

Having said that, there is still utility in being a dick. Since people believe for different reasons, there's no single approach to challenging belief that will work for everyone. A polite, reasoned discourse is best to start with but some people mistake that as validation that their beliefs are worthy of serious consideration and reinforces their belief. Some people believe because they're emotionally invested in it, and they need an emotional approach. But I always recommend against confrontation as the opening gambit.

Finding balance is hard to do, and some people never really find it. When you see atheists behaving like that, talk to them about it and explain it to them.