If that was the case, then every city should be surrounded by enough rurals to feed it twice! 
It *was* intended that food serve as a leverage for war and conflict, this was stated many times. For this to be fully effective, though, there needs to be a greater control over the resource. Otherwise, treaties that concern food will just be too complicated to put in action for people to bother with.
Thus if you want food, war and stealing regions will perhaps be preferable to treaties ensuring food supplies. I've never had a huge problem with this though, the selling party massed the food they wished to sell in one region, set the correct price and a trader or a caravan picked it up. If we were worried that someone would buy it before the caravan arrived, we simply messaged the Lord with the ETA for the caravans. All worked pretty well.
