WOW...Didn't think this practice still exists.... Long time since heard last time someone mentioning this.
I there was a lot of trouble on the Rotterdam in the early 80's. People would leave their items on the chairs and MIA. But the Deck Staff after a certain amount of time would roll the personal items up in a towel and take it back to there deck. Passengers would question the staff where is their items are and they were presented with them back. Making the space available. I would see items on chairs as early as 7-730 during my early walk.
The best system we enjoyed was on the QE2. You would go to the Sun Deck, select a chair to your liking and it would be tagged with you name for the entire time sailing. You even had the Deck Staff watch you to see that you would not get too burnt maybe falling asleep. Also they would offer a beverage like water to hydrate. Yes, there was a minor on board charge but it did work and very well accepted.
The over all thought to this subject is the consideration to one and another being on vacation and sharing the ship with due politeness.
I know we see the domaining of passengers on Shore Excursions, Tour Buses, Clubs/Bar/Discos/Showrooms. Holding seats for others to come with those entering and looking for space presently. Even the staff makes announcements before a show to not hold seats for those not there yet out of consideration.