Just curious: in the US, if you were denied boarding, is the airline obligated to compensate you in the form of cash? Or are they allowed to get away with giving you a voucher for the equivalent amount ("store credit")?
If it's involuntary, you are owed cash, unless you're denied for weight/balance reasons in which case nothing is due, or you're rebooked on a flight within the hour. If it's voluntary, everything is at the airline's discretion.
The core of the problem here, is that when you accept the voucher or miles, then you waive your right to any further compensation. This is due to the fact that you enter into a new agreement with the airline. So think twice before you accept that $50 voucher next time. You could be entitled up to $1300
United's official rules: http://flyerguide.com/index.php/S*UAL/DENIED-IDB