They reject the existence of God, but then clamor to prove they are "moral" - even though the word has no real meaning in a world without God. Any effort to establish a morality apart from God inevitably relies on the religious traditions they claim to reject. Non-violence, universal rights, and cooperation with others are seen as self-evident priciples, and anyone who dares challenge the validity or question the foundation of those precepts in an atheistic system is typically insulted and belittled for his free-thinking. (e.g. when asked for the source of some universal right, a typical answer is, "How can anyone question the universal right to blah blah blah? You must be a liberal commie pinko fag.")
The reason for this is that there is no validity to those precepts; they are hold-overs from the system they claim to have left behind. This then leaves only two options - 1. Accept the rules and therefore the existence of God. 2. Reject God and live in a world without rules. Option number two sounds liberating and appealing, until it is realized that the other guy has no rules, either. Since neither option is acceptable, it's easier to call names than to think about it.
C.S. Lewis nailed it when he pointed out the circular reaonsing - that selfishness is "bad" because it harms society, and harming society is bad because it's selfish. The atheist will resort either to "self-evident axioms" or throw up his hands and challenge you to come up with a better source of morality outside of God - in spite of the fact that it's his job to defend his belief system.
They may claim to do what's "right" for different motives, but they fail to see that "right" is a "moral" distinction that evaporates in a world without God.