Plain and simple they should have upped their offer and someone would have taken it. The customer was peacefully sitting in his seat and UAL assaulted him. So much for the customer is always right. They could have called him belligerent, let him fly, then banned him from future UAL flights as punishment. No one deserves to be assaulted like that unless they are resisting arrest and he was not being arrested because he broke no laws. If he had, they would have handcuffed him and read him his rights.
I think these were incompetent bullies who had no idea what to do. The fact he ran back on the plane tells me this. It is also being speculated that they sedated him when he was in the back of the plane because he ended up passing out as he walked off the second time. What an absolute mess.