I don't think it's a strawman. When you corner a theist on this point, she will often claim that god provides objective morality regardless of the existence of hell (or other punishments). They really are claiming that without God, morality disappears--hence the relativists can't justify anything schtick. And hence the standard non-answer to the Euthyphro dilemma, as opposed to simply admitting, "Yes, it's above him but he enforces it."