Baggage on unoccupied seats

Why can't SWR make it a rule that people should not put bags or luggage on unoccupied seats.

On a packed commuter train from Waterloo to Portsmouth, people are standing in aisles and sitting on the floor and one man is putting up a struggle when asked to move small items off seat onto luggage rack.
It took two people to combine forces to get him to move his stuff so a lady could sit down.

I believe on New York subway it is illegal to put bags or other stuff on seats. All it would take is a sign and regular reminders on tannoy during other routine announcements. Can this be done?