I've always thought this, every since I was little and being literally forced to eat. If a child has eaten as much as they want, and are saying they are no longer hungry, isn't it still "food waste" to force it down them? Likewise, overeating just because you've made (or someone else has made) more food than is needed and don't want to throw it away - surely that's still wasteful? The only way to avoid wasting food in that scenario is either to package the leftovers hygienically and use them later, or cook less food in the first place.
YABU - if food has been cooked, it should be eaten rather than thrown away whether the diner wants/needs it or not
YANBU - cramming in food that you don't want or need is as much "food waste" as throwing it in the bin