Some (most) of you people are blinded by your emotions. The captain of that flight is the ultimate authority and he asked the cabin crew to carry out a company command (remove 4 passengers) for WHATEVER reason (in this case, to take on 4 non-revs). He can do that without having to justify ANYTHING to ANYONE.
Going back to the very start of this thread, it isn't about what the captain can do. It is about how this situation was managed. It wasn't managed very well! The captain (who answers to a lot of people) is a senior manager in a customer focused industry. That responsibility in turn devolves to a number of other people under his or her charge. Good management involves awareness, common sense, flexibility and communication. When those ingredients are best employed this sort of situation simply shouldn't arise and of course it rarely does. There is no suggestion in any of these reports that the captain was particularly instrumental in managing the sequence of events and it is still sufficiently opaque as to exactly who was driving these decisions. Nevertheless it seems to be have been very poorly managed at many levels.