why do you want to pressure people into eating more?

Well, because that's what the problem statement asked for! But yeah, it's probably a forgotten purpose: what should be optimized is the amount of food not wasted, not how much food remains at the end of the party.

the party-leftovers issue doesn't strike me as much of a problem

It's not indeed! But it's a nice simple little world, I took it as an exercise in rationality.