No idea whether it's a fed'l law, FAA policy, or just something the TA's make up @ time of check-in, but I can vouch for being not only asked whether my furpeople could stand up in their carriers 2 yrs ago when we moved, but the TA (on AA) actually wanted to see each of them (cats) do it. I plunked the male's carrier on the counter & told her to discuss w/him directly but that he was in a bad mood & then showed her my ripped up arm where we disagreed over his getting in the carrier in the first place. She didn't mention it again.