It may have been legal, but there is an old saying; “You can be right, or you can be happy”. United chose “right”, and now they are not happy. Tough: They can just deal with it. My guess is that for the near future they will have some extra seats available. Just in case they need them for higher priority passengers.
In other words right or wrong matters not if it doesn’t make you happy!Now I know why rioters riot.