Elon Musk has an obvious technical competence that's very appealing to those who appreciate those skill sets. It's like Paul Graham -- when you listen to him speak, he's mostly soft-spoken and measured (I have no explanation for Musk's Twitter, which seems much more... hyperbolic). It's clear to me this isn't charisma exactly -- he doesn't strike me as charming, just like he cares a lot about understanding his businesses in tremendous detail.
You're absolutely right that this shouldn't matter. But since charisma _does_ matter, the SEC just has to paint him as an angry anorak trying to punish short sellers. I don't think that picture is hard to paint (I can't see into his mind, so obviously I have no idea if it has even the smallest element of truth).