This seems really, really odd - what's going on?

What you'd expect? The usual: half educated people making basic errors, not making sure their decision theories work on 'trivial' problems, not doing due work to find flaws in own ideas, hence announcing solutions to hard problems that others don't announce. Same as asking why only some coldfusion community solved world's energy problems.

edit: actually, in all fairness, I think there can be not bad ideas to explore in work you see on LW. It is just that what you see normally published as 'decision theory' is pretty well formalized and structured in such a way that one wouldn't have to search enormous space of possible flaws and possible steel-man and possible flaws in steel-man etc, to declare something invalid (that is the point of writing things formally and making mathematical proofs, that you can expect to see if its wrong). I don't see any to-the-point formal papers on TDT here.