No it isn't. The club don't sell the food or drink and don't set the prices. It also isnt up to you to tell people what they can or cant buy at the ground. People can make their own decision - it is a free world. Inside the ground it is all about convenience, which has a value. I am sure they would let you outside to go and buy something if you asked nicely. You might miss 15 minutes of the game but worth it to save a pound or two (not).