Looking at it the other way, I'd be mightily dischuffed if I'd found myself what looked like a nice unreserved seat, only to be turfed out later by someone who'd actually reserved it - especially if the genuinely unreserved seats had all filled up in the meantime! I appreciate the reservation holder's frustration in that case, but really think that a free for all is the fairest approach!
Been there, had that argument, and refused to move - to much unhappiness from the aggrieved party. I did explain that he could apply for a full refund of the fee that he had paid for his seat reservation as it had not been honoured....
The train was from North Wales to London with no electronic reservations showing or cardboard labels inserted. Reasonably busy leaving Llandudno Junction, full and standing when it left Chester - where the reservation situation arose.
I had sympathy but, at the end of the day, there were no reservations displayed. The person who had reserved the seat was younger than me and had no (visible) need for the seat. If it had been an elderly, frail, pregnant or clearly disabled person then I would have been straight out of the seat, even if they had had no reservation - merely a greater need than my own.
On the other hand, I've boarded TPE trains at Manchester Piccadilly heading for Scotland (with a seat reservation) and not bothered even trying to fight my way through the packed train to see if someone has sat in my reserved seat. Generally, you get a seat eventually after Bolton or Preston calls.
On the subject of the thread. I was on a Virgin train south from Piccadilly last weekend. I sat in one seat of four around a table. A young woman sat on the opposite seat and filled the adjacent seat with her stuff - nothing bulky, merely to fill the second seat. A man arrived, asked her politely if he could sit down, she simply picked up all her stuff and moved further down the train. Clearly she simply had to have two seats to herself!
Nowt so queer as folk.