A couple of thoughts on some of the other comments made-- I've been to some (frankly, mediocre) all-you-can-eat sushi restaurants where you pay a flat fee, and then also pay for what you don't finish. Because sushi is expensive, and someone else could have eaten what you threw away. I thought that was interesting as an incentive to not take as much from the buffet.
I have also been to restaurants where sharing was discouraged with extra charges. My guess is that the restaurant wanted to ensure that a couple occupying a table for 2 hours would eat for 2 people rather than 1, which would result in lost profits.