I doubt there's any reasoning behind it beyond "but they're people!"
I find the comparison between the treatment of mentally handicapped people and smarter animals to be a very interesting moral issue. I've not seen an argument that doesn't essentially boil down to "but humans are better".
This subject is the topic of Giorgio Agamben's "The Open" (2003), which is a short (though dense) volume. Definitely worth a read if you are interested in the subject!
Its probably less about the handicapped person and more about everyone else. "Less handicapped" might factor out to smarter, stronger, male-er ... whiter... it gets ugly fast.
Its comforting to know that human rights are a binary function and not a sliding scale, even if you'd end up on the top end.
I find the comparison between the treatment of mentally handicapped people and smarter animals to be a very interesting moral issue. I've not seen an argument that doesn't essentially boil down to "but humans are better".