I'm not convinced there is no theory of a just riot, at least inasmuch as the majority of riots in history seem to have non-participating academic types willing to explain explaining why the anger was in this case justified. (Some of it's pretty ad hoc rationalisation, but lots of "just war" claims are too). Plenty of theory about how much rioting is too much rioting too.

It's just that most people who believe a cause justifies a riot (whether that's police killings, blasphemy, a candidate not winning an election or high taxes) want the riot to go beyond a riot and into some sort of actual change. A war is justified as a means to an end; a riot is usually only a small part of that means.

Also, a lot of just war theory boils down to asking religious figures or bodies like the UN for permission, which isn't really practical for your average riot.