I think it's amazing that a PI was hired, it's crazy. If these guys were doing stupid stuff with their FMLA I don't know what they were thinking, but right or wrong they got fired and will be fighting to get their jobs back.
I have to wonder about the PI, though. This is proof that the company is going to some pretty serious lengths to dig up any infraction of the rules, but how many people are on FMLA? What other people are going to get a PI tailing them?
Bad enough you need to watch your back at work, but now you need to watch your back at home, too.