Health and safety. The food served at the canteens needs to be traceable, to avoid/mitigate food poisoning related issues. Once you start bringing external fo

Health and safety. The food served at the canteens needs to be traceable, to avoid/mitigate food poisoning related issues. Once you start bringing external food into the system, you can no longer guarantee the safety of the other canteen goers, even if, theoretically, they will not be sharing their meals.