Seems so strange to me~the appetizer for two is between $2-$10 LESS than the entrees!
Don't expect it to make sense! By that logic, they should allow cheap appetizers to be substituted for desserts.
From what I can piece together, from bits of information here and there, and from my own experience... (not sure how true this is, but this is what I think is happening)
Especially as to non-Disney-owned restaurants -- Everything is heavily computerized. The non-Disney owned restaurants especially need to be sure they get the reimbursement from Disney for your meal. If the don't enter a properly coded meal into the computer, they won't get their reimbursement. The meal coding has nothing to do with price, but about picking exactly 1 entree, exactly 1 dessert, exactly 1 beverage.
Now, they *could* get creative... enter an entree code into the computer, but actually serve you the Dim Sum for 2. But not sure how willing they would be to do that.
Even if you were on the deluxe plan -- You wouldn't be able to get a "for 2" appetizer, unless 2 adults pooled their credits and shared it.
So yet another reason, why it wouldn't be proper coded for a single individual.