I thought the FA was actually referring to the bar under the front of the seat (in this case, 10A/F) which prevents the bags from sliding forward. If it were a bar to keep them from sliding backwards, well, I don't know that ANY exit row seats (or seats, period) on UA have such a thing.

I've personally only had one UA FA ever make the claim that a far-away exit row's seat couldn't be used for under-seat stowage (was an Airbus, IIRC). I try to get these seats whenever possible, too. I have had more problems on US--such as the FA telling me I couldn't have a pillow in the exit row (), which this being FT, when I posted about it, someone replied in all seriousness, "Hey, I don't want your pillow getting in my way in case of an emergency." (double to that.)