There is a very big reason why I do not fly United. When I was flying in the military and I flew a lot I would avoid United after two bad situations with them. I fly American as much as possible anymore they have never treated me wrong or another passenger as far as I can tell.
I've been keeping up with this since I've been flying a lot recently. I fly either Southwest or Delta, and am glad I didn't choose United. Luckily people's protests seem to be having an effect, considering I just read that United's shares have dropped by 6%.
Don't forget, legally the airline can tell you to give up your seat. Giving the voucher is really optional. While we are looking at the airline we should also focus on the fact it was airport security carrying this out and not the direct airline staff.
Removing a passenger is one thing, I think the problem everyone has is HOW their security removed the guy. Beating him unconscious and slamming his head into a headrest is a little extreme. Regardless, they overbooked and airlines usually won't let you board the plane if they know they won't have seats. This is definitely on the fault of United because there are so many checks and balances that should have happened long before they got to this position.