A Contradiction in the Lives of the Rational Responders

I have recieved repeated threats of lawsuit now from the Rational Response Squad and their supporters for using the image of the shield as my picture. I cannot help but notice an inherent contradiction. Their metaphysic is opposed to their moral conclusions. Can somebody please explain to me how a naturalist is justified in saying that it is wrong for me to do anything? If God doesn't exist, then neither does an objective morality. But if an objective morality doesn't exist, then all we have are a bunch of arbitrary laws put forth by governments. Now an atheist may say that we should obey these arbitrary laws, but why? If there is no objective law stating that we should, it is a mere case of opinion. This, my friends is very common within this debate. An atheist must borrow from a theistic worldview to make his (or her) points and demands. Atheists... I offer you another challenge. I will gladly take down the image if you can convince me, by reason, how a naturalistic world produces "oughtness."