In terms of food, I would find it very unlikely that the child is queued up to purchase something without an adult involved in the process.
Yes, but an adult without children getting in line means that adults with children (and by extension, the children) will have a longer wait.
The difference is that there are many, many rides that are not only appropriate for adults but that kids just aren't allowed to go on.
MK - Space Mountain
EPCOT - Mission Space: Orange
AK - Expedition Everest
AK - Flight of Passage
HS - Rock n Roller Coaster
AK - Primeval Whirl
Not counting the water parks, three rides at Animal Kingdom, and one each at the other three parks.
So this is an odd philosophy to me and I wonder if it's just applied to children? Like there are rides that are just not suitable for people in wheelchairs, should able-legged people refrain from getting in line at the other rides? Instead of making the line longer for handicapped people, the rest of us can just ride the few attractions wheelchairs just don't get to?
Your position that children deserve exclusive access to the character meets is troublesome because it assumes that children enjoy the characters more than adults do. That's an impossible determination to make on both ends. I'll grant that a parent probably knows how much their child likes to meet the characters, but how ... by what metric do you compare that enjoyment to the enjoyment of a stranger, regardless of age? You can't; not with a straight face anyway.
This is my guess, and I'm not trying to roast you here but it's the only way this fits. You know how much your kids enjoy meeting the characters, your kiddo still wants to do it at the near adulthood of 12. And you know how much you like meeting the characters, quite a bit less it seems. And you consider both the kids you know and yourself to be more or less normal specimens of the species, it's a short leap to view the rest of us through that lens.
It's developed into a question of morality for you, adults who put their enjoyment of this particular sort of Disney attraction ahead of the enjoyment of other people's children are being discourteous. This is a dangerous form of judgmentalism, because in real life people are less likely to be kind, forgiving, helpful, charitable, all around good people to others they view as discourteous.