Well, that may be, but two weeks ago, I volunteered to take a flight the next day (domestic) and my bag went on the scheduled flight without me. It was sitting in baggage claim when I got to my destination the next day.
And I had this happen earlier this year too. So, no, United does not always pull bags when a passenger voluntarily takes a later flight....
It is true that it was not my intent to take a later flight - and United was the one who decided it was me taking the later flight - as would be in the case of an involuntary denied boarding.
But it takes time to find one single bag in the hold. Many bags look alike - last time I actually entered the luggage hold (a long time ago) it was a jumble of bags, and dimly lit too.
Post 9/11, I have also witnessed a flight attendant who left the flight minutes before the flight got clearance to leave (we were late, and she (unwisely) decided to leave the plane to buy some food). This was an international flight. Infuriated, the lead FA and Pilot decided to throw her bag off the plane and leave once clearance was granted. As far as I know, she only had carry on luggage. We left one FA short.
Billy