The first question is whether you can have rent-a-cops throw a passenger off a plane after they have taken their seat. As I understand it, that is not legal.
The second question is about over-booking more broadly. After all, though there's a legal distinction between a passenger a second before boarding and a second after, from a customer service standpoint they're still denied the seat they were promised. Is it right that an airline can sell 310 tickets on a 300 seat plane, deliberately entering into 10 contracts it knows it cannot honour? As I understand it, that is legal. But should it be?