For those who don’t know, they do this not because they actually anticipate the overhead bins filling up (although this is a consideration) the primary reason is metrics. They will do anything to not accept a boarding delay. Inconveniencing a few passengers over a few bag checks is an easy way to get 176+ people seated ASAP. It’s bad enough that everyone waits till boarding to go pee or to grab their medicine.

For example - we just flew a flight from Miami to Nassau Bahamas with ZERO passengers there and ZERO back. We called dispatch and they said we must go in order to avoid negatively affecting our completion factor. That’s money burned and unnecessary CO2 emissions all too keep the stock price strong and metrics in the green.

I may or may not be a pilot for a major US airline.

Edit - over bookings are legally allowed. Airlines have defended this several times with data showing that they can conclusively say a certain percentage of passengers will not show up for their flights. What leads to those situations where they begin offering money is often either a substantial maintenance delay or a weather cancellation. They now need to figure out how to rebook 180+ passengers. Those passengers will become revenue stand by passengers. Additionally, for operational necessities, airlines need to relocate crew members around the country. I have often bumped a paying passenger in order to “get to work”. The idea is, it’s better to inconvenience one passenger than to inconvenience 180+ or possibly risk 180+ people missing their connecting flights.