Also, with all the recent "purges" of old school senior military officers in all branches, a steady flow of career-climbing, PC-infused successors have risen up to impose the Left's idea of military service onto the ranks. It's one of the reasons my buddy is hang it up at the end of the year---he can no longer stand the latest incessant PC indoctrination that is changing the mission from warfighting to liberal social engineering. He's just had enough of it.
It was. I don't know if it still is. As I recall, every soldier in the Army received a legal brief that covers topics like UCMJ, Posse Comitatus, etc.., at least once a year, at a minimum. That doesn't mean everyone understood it or paid attention.
Back in the day, before we had the luxury of the interwebs, you had to search out a specific regulation or a field manual to learn about things like Posse Comitatus on your own. There really is no excuse not to understand that constraint today, given the technology to retrieve the information with a mouse click.