I can understand. For someone who can walk a few steps and get in and out of the ride cars with little assistance, they can pretty much ride everything.
But, for someone who can either not be out of their wheelchair, or has a very difficult time getting out, there are fewer things they can do, especially at MK.
When my DD was little, we could lift her out of her wheelchair and bring her onto most rides. Now that she is bigger it is harder to get her out of the wheelchair and into the ride cars that require a transfer. Because MK is older and less accessible, there are few attractions there that she can go on - in Fantasyland, for example, shecan go on the Winnie the Pooh ride, Small World and Mickey's Philharmagic. The other attractions in that area require much too difficult transfers.
Sea World recognizes that some people are not able to experience all their attractions because of disabilities and do offer a discount, so I can see why you would ask if WDW does also.