I can see Disney charging extra for special requests. I believe that the food is created in mass quantities now due to the popularity of the dining plan and increased volume at restaurants. So, to adjust the smallest thing requires them to make something from scratch.
When I go to a local restaurant here they charge me for just getting olive oil and garlic on pasta instead of sauce...don't know why, but they do. It's their restaurant.
I also see a major increase (and have commented on this on a thread about a vegetarian dining review here and got slammed for it) in prices due to the time the chef takes to discuss special needs.
They don't charge extra for doing it when you have the dining plan because they want everyone to buy the dining plan. They want you to give them all the money up front. They know that a lot of people will leave with paid but unused credits. They also know that most people wouldn't invest as much money in dining if they paid OOP than if they pre-pay and enjoy the convenience (most people aren't ordering desserts multiple times a day, etc.).
So, it is in their best interest to offer you this 'incentive' as a 'convenience' for those who might have special dietary needs and don't want sticker shock on their meal.
But what it comes down to is many restaurants in the country charge for special requests. Whether it is choice or medically related really doesn't matter to them.
I have to use an
ECV this trip to Disney and my special need it going to cost me several hundred dollars. I need a special service now related to a disability and Disney isn't going to accommodate me for free.