Yes UA was in its legal right to kick people off and the man was not within his legal right to object, but...
- UA boarded the plane and then later changed its mine that the plane was over booked.
- They did not use the full amount in offering people to get off
- The man initially offered to go on money alone, but later declined when the timing was suitable and he explained why
The solution was obvious, offer more money and to hell with the $1300 cap. You UA allowed the plane to be boarded UA, YOU F__KED UP. They tried to do a quick and dirty fix and it cost them more time and a hell of alot more money.
They were bloody lucky it was in the USA where the population at large is very cautious about what they do on planes for fear of being arrested. I'm sure in other countries this could have gotten ugly quickly as other passengers stepped in to support the guy.