No, no its really not.
If she was the average size of a teenager, her parents could be hurt and she could be hurt just trying to bathe her. Would you rather her suffer a broken bone than to stay a small, manageable size?
And what about when she hits puberty? What about if someone sexually abused her and she became pregnant? How do you explain that to a mentally disabled person with the mind of a child? Would you want have to deal with her menstruation?
This isn't cruel. This isn't wrong. No one has the right to judge the parents, because none of us are in their situation. I don't envy their position. This is just a sad situation overall.
She's never going to grow up if they do this or not. She has the mind of a child. If she was physically an adult, but mentally a child she has still not grown up. They're not doing this to a normal child. Ashley is a small child in her mind, and will always be.