Assuming you are right about these people, does it really matter? There are MANY forms of morality, from moral relativism to the belief in an objective morality... all of them exist within the moral spectrum where morality is not derived from an authority (I consider the Theistic morality you speak of to be a specific form of authority based morality). That there are places where people with similar moral values gather together is only natural, that does not mean you can draw conclusions about atheists as a group more then you can draw conclusions about theists as a group when all you have seen is one side. I hope that is put in a clear way.I know that. However, I have come into a lot of non-Theists who do have a very "I don't like X therefore X is immoral mindset." It's not all of them. And most of the ones on this forum are not like that. But on another forums I participated in...a lot of the Non-Theists were very "I don't like X and therefore X is immoral." Then when you try to argue they'd say "Do you like X" I'd say "No, I don't" They'd respond "So how can you say that X is moral?"
It was ridiculous and it's part of the reason I left and came here. It's not all non-Theists, but you meet a lot of them who think the same way and you start to wonder.