Surely some people will sit where they want, meaning the system fails?
Your ticket has a seat number. Doesn't matter where you are sitting. If you cause trouble the police will match the seat number on your ticket to the name of the person who bought it and check your ID. If these are not the same, then both will be banned for a year. Really doesn't matter what seat you are in - what matters is that your ID and the ID of the person who bought your ticket are the same.
It's not complicated.