Just Bumped this thread because exactly the same thing happened just in front of me this week and it completely changed my mind and reversed my opinion.
I caught the Brighton to Bristol Temple Meads train. Got a good seat with a table and settled down with my kindle. (Let me say here there were no reservation tickets any where in the carriage). By the time we got to Shoreham the carriage was full. As we arrived at Havant a middle aged couple got on and walked along checking the seat numbers eventually stopping at the pair of seats in front of me and telling the occupants to move as they had booked those seats. The two people did move although they were very pissed off and I couldn't help but think of this thread and even worse I think I'd have told them to get lost as it seemed very unfair to me. Those people had been in their seats from Brighton and chosen their seats in good faith with no knowledge of the problem to come. Buggered if I'd have moved.
Unless they had of coursed booked the seats and had evidence. I'd be pissed off if I took the time to reserve seats and others that didn't then chose to sit in them. Thing is, did they have any evidence they had reserved them?