No, but standby seating is not really something people pursue anymore in this era of cutthroat pricing and overbooking. And then you can only give that seat away at the very last minute, which pretty much guarantees that you will be delaying that plane, with much higher financial consequences for the airline. I think overbooking is a better solution for the airlines.

Nope. Each leg can be oversold, hence their complain is only about the lost money. And control.

Regulations, probably, but I also don't think you are allowed to randomly walk around the airport looking for empty seats if you don't have a ticket, and if I have a ticket I'm just going to my gate, since if I am at the airport it means my flight is about to leave anyway.