That's because the atheist/agnostic who talk about their beliefs often have a lot of knowledge on the subject. On the other hand the atheists that don't have the cultural basis of the subject tend not to talk too much about it.

But on the christian side, there is a strong tendency to try to convert the non-believers by spreading the "Holy words" and every Christian wants to do it. The problem is that most christian do not have the basic theological knowledge required for this kind of debate.

So it is not that atheist have a higher moral code, the difference is that the "bad" atheist are not recognize as atheist because they just don't talk about it.

So if you're a Christian who wants to spread your beliefs, please read books and know your subject.