Imma break it down for you:

• The only reason atheists don't do (X misdeed) is because they either simply feel it's wrong or they don't have the physical, emotional, financial means
• if they don't feel X is wrong and gain the means, they will get around to doing X.
• if they feel X is wrong, that's because of the vestiges of Christian morality they've absorbed from our culture
• however, because each atheist is his own god, they can rationalize themselves past any moral quandary if they want to do X bad enough. E.g. Peter Singer holds that you should be able to kill children up to five years old "if it maximizes happiness."

So to sum it up, let's take an atheist with moral convictions against rape. It physically repulses him, so he doesn't bend his "rules." But the same atheist is strongly tempted to lie for personal gain. He will rationalize that everyone is better off if he lies and goes his merry way, lying.