There is no "they". The military adheres to the law for only as long as those laws are enforced. If the US military can commit war crimes abroad and be protected from outside prosecution, there is nothing stopping them from doing the same inside the US. All it takes is for one person in charge to ignore charges in a military court, or simply not comply with a legal summons in civilian court.

Look at how the US treats the International Criminal Court today, a legal body it itself helped create after WWII. Not a huge leap to think they will ignore state courts in the US. Who's going to arrest them?