How does anyone in their right mind (so far 2 people have suggested) that this is an IDB? The OP removed himself from a flight. Did the airline deny him boarding due to oversales? (a flight that may be overbooked, but for which the OP was asigned a seat does NOT fall under the DoT's or UA's def of IDB.)

The situation was not handled well, but how does throwing out the words "IDB" help the situation? They don't becuase they have absolutely zero to do with the situation. We all know IDB is a term, a regualted thing, with specific criteria to be met. If those criteria aren't met, then how does shoutin it out do anything except distract us from th actual issues? One might as well yell "Fire", "Rape", "Racist" or some other charged phrase that is not pertinant to the case.

If the armrests could not go down, then the large person should have been removed without having pre-purchased a 2nd seat, providing there were no addl open seats in the cabin. There is no way that boarding the number of passengers equal to the capacity of the aircraft is an IDB situation.