The Missing Voice
It's not that people do this both with pets and disabled people that bothers me. I think very often the offense at being 'treated like an animal' is about things that are problematic when done to animals, as well. And that's true in this case, although I'll be discussing why it's problematic to do this to disabled people.
Although one problem is that it can be hard to tell who actually wrote it, that's not the biggest problem either. Often it's obvious - frequent references to 'mommy thought' with too much detail to be anyone other than her, statements like 'I can't speak or write', etc. Other times, it isn't. One website I found, I actually couldn't tell who had written it - the disabled man himself or one of his parents.
In the Faces of Autism conference I went to, they asked people to put up their hands when they named certain groups of people attending. They listed parents of autistic children, teachers, therapists, etc. At the end, they said 'did we miss any group?' I and one other person put up our hands. They'd missed two categories of people - politicians and autistic people. (There's a funny Monty Python skit about this, too. You can see it here*.)
I think these are related problems. People tend to lump disabled people and their family into one group, and act as if their interests are the same. This is especially true with disabilities that are usually diagnosed in childhood and that affect communication. And lumping those two groups together almost always means only listening to parents.
Now, parents of disabled children need to have a voice, and they need to be heard. But that's not a big problem. It's not that hard to get people to listen to them. But so often, people don't even realize someone's voice is missing. They don't even realize that they don't know what the disabled people themselves actually want, how they actually view their own lives.
Back to the 'nonfiction' written from someone else's perspective. The big problem is that they don't seem to realize it's actually fiction. You can't actually write from someone else's perspective, and really get it right. I'd find nothing wrong with it if it came with a clear message that this was written by someone else trying to imagine what it's like from their point of view. I want people to recognize what that kind of writing really is - fiction. It's fiction just like a story from the perspective of a famous person (like the Royal Diaries series) is fiction. Technically, you might call it 'creative nonfiction', which is really a fictionalized account of real events. But it's not equivalent to telling the story from your own perspective or taking dictation.
Another concern I have is that people only tend to do this with those who can't tell you their own story. Imagine writing your husband's life from his perspective - not showing him what you wrote or asking him what it was like for him, just writing it. If you wouldn't be willing to do that, then why are you willing to do the same with your child's life?
This also ties in with the saying to parents: 'you are the expert on your child.' Well, you aren't the expert on your child. You're one of the experts on your child. But the one who really knows the most about your child is not you, but your child themselves. Even if you can't talk to them and find out what they know about themselves, never forget that they do know themselves in a way no one else can. Just because you don't know their perspective doesn't mean they don't have one, or that their perspective brings no new information. And your voice may speak for your child out of necessity, but never forget the voice that is missing.
* Sorry, no subtitles.