That makes sense -- and you raise a good point about this being discriminatory towards disabled people. Unfortunately, though, it seems that the only thing that makes United different from the other major U.S. carriers on this is that it's a $25 fee instead of a $15 one. (Virgin America, Jet Blue, and Southwest still don't charge, but they're also not major airlines and have relatively limited coverage and routes.)

I wonder whether United has some sort of process for accomodation of disabled people so as to be able to argue that this isn't discrimination. I wonder if it would be effective protest to insist on using it.

I also am tempted to note to the relevant carriers that, seeing as how they are feeling it's appropriate to do whatever they can within the limits of the contract to cost me money, I feel no qualms about doing the same to them -- and thus, given that they (or, at least, some of them) do not have a weight limit on carry-on bags, I will make a policy of filling mine with bricks as a form of protest before each flight.