At a high level of abstraction, marijuana seems to make sense to treat autism. The various flavors of autism involve difficulty seeing patterns, and THC makes pattern detection fire more often. While frequent marijuana use can take the brain too far in the other direction (seeing patterns which aren't there, synesthesia, etc.), it seems like there might be promise, though certainly not without side effects.

The only way to be sure is further research, and it's unfortunate that the political and social stigma make it so taboo.