It's the equation that animals are like human beings, while ignoring how non-white minorities are often animalized. That's why the transracial adoption aspect of it is so problematic: that adopting a dog is anything like adopting a human being. Someone in Jim Hines's LJ made the point that animals are not citizens; they don't vote, they don't live independently. Which I think illustrates why it's problematic quite neatly, I think.