It does seem unfair - but I think it is common to all travel and has to do with laws around home security, etc.
I had a flight once where myu wife had prepaid but could not go. She had the seat next to me.
So I said, "because my I paid for that seat I assume you will leave it open for me, so at least I can stretch out?"
Wrong - they said she had to be onboard the plane to claim that seat. They sold to someone again even though I had already paid for it.