OP was a standby passenger. If AS wants to bump him and give the seat to someone else, that is up to AS. Whether it is a good or bad reason and whether AS charges the other passenger or not is also is a business and customer service matter.

Another carrier might have bumped the COS and made him wait for a flight with 2 seats, but maybe the COS had the same status or was somehow an HVC.

AS's system will show OP as a SB and not a confirmed passenger and that is done for exactly this reason. If AS needs to bump OP, it won't offer VDB and it hasn't IDB.

Every US carrier does it this way for this reason. I've been on other carrier's flights where we've returned to the gate and SB's have been pulled so that a "must fly" (crew needed to operate another flight) have been boarded.

I don't know that AS has an obligation to get you your seat assignment back, but you do hold a confirmed reservation on the later flight. So, maybe you've got a middle seat next to the lav, but you've got a seat or some good cash (or they shift someone out of what was your seat).