This is definitely an AA rule, at least at LHR. I wrote about this same issue a year ago because I thought it was not a good (or a revenue generating) policy. It only seems to apply when you are inbound to the states. Once you are in ORD you can standby for just about anything going anywhere.
In my case I was leaving LHR headed to ORD, had no checked luggage, traveling alone, PLT and I went to the airport hours early because of schedule changes and thinking (at that time) I could try and standby. I tried sweet talking the ticketing agent, 2 admirals club agents, and 2 different flight gate agents and it was nope, policy changed a few years back, no standby's without a $250 (as I recall) change fee. So I sat and watch two flights take off before mine, totally wide open seating in Y. Then I boarded my planned flight hours later.
So they got no additional revenue, got me upset over the stupid rule, so I fly less international on AA (ie less revenue). I also wrote AA a letter. So I've accepted it...I don't like it, but that's the way it is and it is well documented in the ticket rules that way.
My suggestion to AA was; allow this for your elites (EXP, PLT and/or GLD's) and use it as a differentiator from non-elites. That might in fact increase their revenue and elite goodwill, which we all know definitely increases revenue.
