I have been hit by Ryan Air and other European low cost airlines on carry-on's. Often paying more for the carry-on than the cost of my ticket.

I doubt American Airlines is moving in that direction. They should just enforce the carry on limits and check those that don't fit it.

From the revenue perspective, I'm sure they will lose a lot of kettles. AA is not in that positions right now. So I will call what the OP posted, a programmer's mistake.