Personally I'd have thought setting up their own matchday catering service would have been in breach of contract but obviously not.
There's plenty of chip/burger vans park over the bridge - they also take revenue away from the stadium owners. The Brickmakers arms takes revenue away on latics match days.
There's a limit on how far the stadium owners can go with regard to controlling where fans can eat and drink before kick off. They already get all the profits from inside the ground, and if they upped the rent for the warriors, more fans would boycott the inside facilities in favour of spending them in outlets that support the club they support.