Except that he still does, in the first line: "It is not /enough/ for me to stand before you tonight and condemn riots." He is doing /more than/ condemning the riot.

One can condemn a riot, condemn misbehavior and hatred, while still acknowledging and moving forward in contending with the real and terrible conditions which provoke riotous behavior in the first place.

What MLK is saying is that you can't just placate people by saying "Rioting is bad, stop it."

You have to actually address the problems that lie at the root of the riot.

But that doesn't mean the riot is an acceptable course, merely that the riot itself is a symptom of a deeper issue rather than the disease itself.

Related:

"But it is not enough for me to stand before you tonight and condemn riots. It would be morally irresponsible for me to do that without, at the same time, condemning the contingent, intolerable conditions that exist in our society. These conditions are the things that cause individuals to feel that they have no other alternative than to engage in violent rebellions to get attention. And I must say tonight that a riot is the language of the unheard. And what is it America has failed to hear? It has failed to hear that the plight of the negro poor has worsened over the last twelve or fifteen years. It has failed to hear that the promises of freedom and justice have not been met. And it has failed to hear that large segments of white society are more concerned about tranquility and the status quo than about justice and humanity."

- From The Other America