No airline does.
Why? Do the math.
Invol compensation is EITHER 4x the one way fare or $1350, whichever is less.
This flight was domestic to kentucky. After offering $800 for a vol bump and not getting enough takers, the gate agent’s computer invol bumps the lowest paid ticket on the plane, and lets say it is $200.
That means united has to pay that passenger $800 for the involuntary bump.
The fact that the man refused airplane orders to get off the plane (which is required by law) is not United’s fault. Nor is the fact he got injured, which is some unknown combination of his own fault and the police. He would have at least partial responsibility for creating the situation he suffered by refusing legimiate orders.