Discussed before, but irrelevant, because he wasn't "denied boarding." He already boarded.
You must be a lawyer; to the rest of the world not being allowed to board and being slung off just after you did board are pretty much the same thing.
The alternative course of action to forcing people off an aircraft by bodily hauling them off with a posse of thugs is to simply raise the ante until sufficient "volunteers" are found. Compared to the cost of delays, bad
PR, buying alternative flights, etc etc there is almost no figure that is too high; if it had cost $40,000 to get 4 people off that UA aircraft it was still the cheapest solution. And trust me, I could have got 4 volunteers for a lot less than that.
Overbooking was, let's remind ourselves, a necessary evil in the good 'ole days that I knew, when a no-show was accommodated on a later flight, thus losing the carrier some money. Possibly.
Nowadays, THERE IS NO EXCUSE. If all the seats on the flight are sold, they are all paid for, no refunds allowed for a no-show. There is no need whatsoever to overbook to protect the revenue. Overbooking is simply a nasty con-trick to get 110% (or whatever, you know what I mean) of the seats paid for, most of the time. Regulators should get on top of this now, and stop placating the operators by ignoring it.