"UA could have made the offers before boarding the plane."

BINGO! 

It's very common for the other airlines to offer the vouchers BEFORE boarding. I've been bumped a few times at various airports and gladly took the sweet deals. Just recently a different major carrier offered me $1,500 cash, a free car rental and a night at a nice $390 a night hotel BEFORE anyone was allowed to board as long as I agreed to go standby and try to catch any of the next flights the next day.