As some employee posters have noted, overbooked does not equal oversold. Overbooking (selling more tickets than available seats for a scheduled flight) is common; overselling (more people checked in than available seats) really is not, at least in the context of the number of American air travelers every day. It seems unlikely any online check-in alert, or kiosk alert a couple of hours, will correctly identify an oversold condition.

Notifying passengers about overbooking certainly serves a purpose: it allows people to think about it and prompts people with flexibility to self-identify.