Well we can all agree that this is a PR disaster for United and I hope they pay dearly for this. Paying customers already boarded with a seat assignment should not be forcibly removed to make way for staff that they didn't account for properly. United really screwed up and I hope the passenger sues them for millions. The airport police should also be charged with excessive force for the role they played. 

It also points out that an aviation bill of rights needs to be adopted both in Canada and the US, just like in the EU. They should be offering whatever customers are willing to accept as compensation if they are severely in need of seats. If the compensation price is $10,000 then so be it, it is the airline's fault for not planning their network properly.