I can't understand how you cannot accept someone leaving food. Have you never been for a meal and left part of it,either because you are full or not keen on the taste?
But this wasn't part of it - it was almost all of it. And it's not as if it was some exotic dish they were being daring and trying out; it was a slice of cheesecake, a coffee and a muffin. What can go wrong with that? Also, when you stop in a cafe when out shopping and order something like that, it's normally because you want something to keep you going for a bit, so even if it isn't that great, you have at least a couple of mouthfuls, if only so you don't have to stop again half an hour later.
I agree that if you've paid for it you can do what you want - I just don't see the point of throwing money away. Why didn't they share a cake?