UA as any other Carrier has a procedure in place when these things happened.
Step 1 ask for Vols which usually in most acses solves the problem, last yr my IAH-JFK flight that I was protected on (I missed my flight since my plane went MX @ FCO) and yield me a voucher for $500 also for weight problems , was given a Hotel room for the night and was 1 very Happy Camper
Step 2 if step 1 doesnt yield the needed amount of people then its IDB time, in which case you get $$$ and not Monopoly money,( but I dont mind the MM as I need to buy enough tkts every yr to have flown 100k). Im not sure of the process but I do believe that its suppose to be the last people that checked in are those who wont be able to fly. I think that agent in BTV wasnt following proper procedure if in fact they refused boarding to anyone based on the Fare paid
