Food in con is a way the hotel makes money so they don't allow much from outside, even the hot water for some past events has to come from the hotel. Or if you buy some food from out side the con and you bring it into the lobby to munch, yeah they don't like that.
I know in the past it was EXTREMELY hard for neko pan to get in to sell food. They were allowed because of 2 reasons, and even then it came down to right before con they were allowed.
1 They were selling something not available in any form from the hotel space eateries, gourmet specialty cupcakes from scratch - which is why uwajimaya booth can sell a few foods as well
2 Their cupcakes were made in a licensed bakery. That's why every now and then when someone wants to just make cupcakes to sell at con they can't because it has to be in a licensed bakery
I really miss neko pan too. Every year they were at con they gave me a free birthday cupcake. Since most of the time my birthday is during con, it would be the cake I got on my actual birthday day
But yes while part of con is the fun of eating out, a few well planned food purchases can help, some fruit, bread, bagels, easy stuff. I buy the 6 packs of bottled pop when on sale and save it to bring to con because its easier to deal with then a can, and loads cheaper then the vending machines.