Dave/Lex,
I agree the armrest SHOULD stay down...

the airlines sell PASSAGE for one person from one place to another. One person, one ticket. If they don't want to sell that, they need to be clear and specific about what they will transport in one seat... i just can't stand this arbitrary and humiliating treatment some people get... it's so wrong.

For example, they could say in their rules something like: one purchased ticket entitles the passenger passage between two cities... the person must be alive, not more than 6'4" tall, not more than 200 pounds, not more than a 44" waist...

of course what I prefer is the mandatory weight/volume displacement test some clever person proposed earlier... and there could be a sliding scale for the amount of space you take up.

You seem very bitter, Dave/Lex... and if you feel so strongly about it, why don't you just go up to people on the street you think are too fat, and tell them to their face.