The divine command theory of ethics is the idea that nothing is right or wrong outside of God's will. If God wills something, then it's good. If God wills that something not be done, then it's wrong. Good and evil are not independent of God and God's will. Can we really say that this is a genuine theory of morality, though? If not, then divine command theory isn't merely a wrong theory of morality, but isn't even a legitimate theory of morality in the first place.
Divine command theory is closely associated with theism, naturally — there is no such thing as divine command theory without a prior belief in and commitment to sort of theism. Not all theists actually accept this idea of morality, though; in fact, some actually reject it as contrary to true love of God. G. W. Leibniz (1646-1716) wrote in his Discourse on Metaphysics (1686):
In saying, therefore, that things are not good according to any standard of goodness, but simply by the will of God, it seems to me that one destroys, without realizing it, all the love of God and all his glory; for why praise him for what he has done, if he would be equally praiseworthy in doing the contrary? Where will be his justice and his wisdom if he has only a certain despotic power, if arbitrary will takes the place of reasonableness, and if in accord with the definition of tyrants, justice consists in that which is pleasing to the most powerful? Besides it seems that every act of willing supposes some reason for the willing and this reason, of course, must precede the act.
It's hardly surprising that arbitrary and despotic powers have had little problem with the divine command theory of morality. At the very least, getting people accustomed to following divine commands regardless of their consequences or reasonableness — and always thinking that this is "moral" — is a wonderful means for priming people to follow the commands of any other leader as wel — especially one that purports to speak on behalf of or in defense of traditional religion.
Once morality is equality with obedience and sinfulness with disobedience, rulers have taken a major step towards acquiring the unquestioning submission of a significant portion of their community. It's simply not common for religions to promote the questioning of their gods, religious institutions, religious traditions, or religious authorities.
Even though progressives and rebels have always managed to find license for their activism in their religious traditions, the weight of traditions, leaders, and institutions have tended much more strongly towards support of the status quo — including the arbitrary rule of despots for whom consequences and reasonableness are irrelevant.
In the book Toward a New Political Humanism, Theodore Schick comments on Leibniz's observation about how unreasonable it is to follow commands simply because they come from a powerful authority rather than because they are based on clear, reasonable principles:
If no moral principles exist apart from God's will, God's moral choices cannot be principled, and a being that makes unprincipled choices... is not a being worthy of worship. ... If actions are neither right nor wrong independent of God's will, then God cannot choose one over another because it is morally better. Thus, any moral choices God makes must be arbitrary. But a being who acts arbitrarily does not deserve our praise. So not only is the divine command theory implausible, it is impious as well.
It's curious that theists would be consider defending such a position. Who would praise a god for doing something completely arbitrary? Wouldn't worship of such a god be nothing more than the worship of power — the worship of a being merely because it is powerful enough to do whatever it wants? It's strikingly similar to toadying to a powerful king or other ruler who holds people's fate in his hands.
What's especially ironic about this is the fact that atheists are so often accused by conservative Christians of not being able to offer any reasons for being good and instead merely following the arbitrary whims of society, culture, or rulers. It's theists — or at least theists who subscribe to the divine command theory of morality, which tends to include conservative Christians — who are the ones following arbitrary whims. In the end, they also can't offer any good reasons for doing so — just the prudential "reason" of having to avoid hell.