To be clear, I 100% disagree with any use of military action within the States against American citizens.

That being said, what you are saying isn't true. Exceptions to Posse Comitatus include "Federal troops used in accordance to the Insurrection Act, as was the case with the 1st Marine Division and 7th Infantry Division being sent to curtail the 1992 Los Angeles riots."

Should it be illegal for the President to send troops against citizens to restore order? Perhaps, yes. Is it illegal? Certainly not.