Absolutely agree with some comments above that this is unacceptable. flyingtonight i don’t know all the regulations or what airlines teach but would be very surprised if this was the case that they are teaching the method you describe.
In my eyes there are 4 opportunities to visually check and correct seating..
1. check in staff
2. gate staff
3. cabin crew
I would hope that if explained to the passenger the reason that they are unable to be seated in their pre booked seat, having paid or not, would understand based on safety reasons.