[edrtfgdr]

Someone occupying an otherwise empty chair has zero marginal cost. In fact to a restaurant, people at tables has a negative cost just before the busy period - would you pick an empty restaurant? People in a starbucks at non-peak hours aren't a problem and may even attract other customers.

[ibsulon]

power from an outlet, higher electricity bills for cooling (more people, more heat) -- Hypothetically, this would be counteracted by people buying more drinks, but how much sugar can one consume, really?

Myself, I'd suggest some low calorie drinks people can drink to keep going.