Re: UA
Rules are you can be bumped for a flight
https://www.transportation.gov/airconsumer/fly-rights
That said, UA could not have handled it worse and should have taken care of it before he got on the plane. This sort of thing happens daily, usually no one ever hears about it, because airlines generally do a good job of it. I have taken a voluntary bump a few times when I was in no hurry to get where I was going and the $ off a future flight was worth it.
They did nothing wrong bumping the guy .. how they executed it was piss poor beyond belief ... that said, they usually handle it well and this does not warrant some massive overhaul some are asking for.