Suppose the police/security people had criminal background info on this passenger? Would you view their actions in a different light?
Rent-a-cops? That's a real stretch.
Nope. I would not. In fact it would alarm me even more because I would assume they were going out of their way to target a person who from all evidence had satisfied any court actions against him and had not gotten in trouble since. There was certainly nothing to suggest he would be a threat to the airline and its passengers. It would disturb me that without cause and without any legal authorization they had invaded the privacy of a passenger in order to make up for their own failure to provide transport to crew members in a timely manner.
UA screwed up badly on this one.