That isn't always true. An example I can think of is when I am on a paid F/J fare, and I have to take an earlier or later flight because of change in schedule, flight delays, etc. If F/J is full, but there is a guaranteed Y seat, sometimes I need to take the Y seat to get where I need to go on schedule. It would suck having to fly in Y on a paid F/J fare knowing there was a seat available but it was given away for free. So, yes, United gets to keep the revenue regardless, but I end up paying for a service that I didn't receive.

I know this is an isolated example, but it happens a few times a year, and hopefully this was never the reason I missed out on flying the fare I paid for.