If they had just increased the compensation passengers would have voluntarily given up their seat, everyone has their price. It didn't even need to be money, free flights, upgrades, it costs them next to nothing, especially when you consider this is still running in the media days later. Everyone hates to hear they overbook flights so you'd think when they get bitten United would take some of the savings made overboo…
One of the reports I heard from interviews of some other passengers on that flight was that the United issues voucher in $50 denominations, or 16 $50 vouchers expires in a year and have lots of restrictions and fine prints. In other words, the offer was NOT worthy the face value and hassle of "re-accommodation".
The airline may offer some alternative compensation (and of course one can accept it) but you have a right to a check.
Of course the airline may try to hide this from you, or give you a hard time, but according to the Dept. of Transportation you're entitled to cash.