FTW, what the NM's Governor & AG thinks is interesting but is legally immaterial. I haven't read that the AG is charging any of the militia with false imprisonment, assault, or brandishing a weapon. Those are the usual charges laid against overzealous security guards. These charges are at a state level. So, if the AG is so concerned with the legalities of this group's actions, he has plenty to charge them with. (But, he hasn't.)
RE: the Fed. Yes, the leader was arrested on firearms violations. But, kidnapping is a big-time Federal crime. So, if there was any prosecutorial substance to the kidnapping charge, the FBI could and would have brought those charges in a heartbeat. Again, the ACLU's comments have no legal standing.