Why do people think that if an omnipotent entity exists, therefore objective morality exists?

I've wondered this many times. I don't think that it's because god can change his nature though. Even if god's nature was fixed, that wouldn't mean that whatever god thought was good, was necessarily good. I mean, god might be a complete b@st@rd, just because he has the power doesn't make him and his way of doing things good. The Nazis had the power but how many people think that that made them right?