My DC are relatively unfussy, and love their food, btw. Except for DS3, but he's only 16mo and has the appetite of a sparrow.
It's not about fun or not, it's about cost and waste. I can't afford to do lots of separate dinners, I am on a very limited budget, and most dinners are 'all in' dinners, like slow cooker meals, so that I can buy cheaper cuts of meat.
I don't expect a totally empty plate to get pudding, but they know they have to eat a reasonable amount of dinner to be allowed pudding. I don't think it's that odd, tbh, don't know that many people in RL who would allow their DC to be that fussy.
One thing you accept if eating in someone else's house is that the food may not taste the same as it does at home. I personally am fussy about spag Bol, and won't eat it if it is from a jar. If I am offered round a friend's house, I will politely decline and say that the meat for dinner needs to be eaten tonight or something, as a polite get-out. Anything else though, I will eat even if it is horrid, as my friend would have gone to the effort of cooking for me. And so would my DC.
There's nothing wrong with having certain expectations of politeness from DC when out visiting, and I would expect a DC of school age or almost school age to understand the expectations I have of politeness when visiting someone.
When I said that the DC sounds like a spoiled little madam, I wasn't being rude about the DC, after all, she can't spoil herself, can she? It was because I get annoyed at parents that pander to their DC like this, having boundaries and expectations on your behaviour never hurt anyone, did it? Not if it's done properly.