They enforce it on connections, too. I had a miserable late departure on AA from HPN in a jungle jet where they took forever to re-distribute pax so the weight would be even- took off an hour late. Of course we arrived at the end of one terminal in ORD and my connection was at the end of the next one. I had carry-on only and ran through the corridors, arriving 5 minutes before scheduled pushback. No dice. Door closed. Game over. I'm sure they were able to put a standby in my seat, so they left without me.