P.J. Denyer
Penultimate Amazing
I'm not sure if the assumption is that they cannot be so much as they won't be. Society doesn't want the hassle and expense of doing that rehabilitation for the owner, and we think the owners are likely to continue to fail to keep the dog under control (which is often true), so just for expediency we remove the problem in the surest manner available.
Harsh but practical. The distinction isn't so much that dogs are less able to be rehabilitated than humans, but that they're worth less to begin with and so erring on the side of convenience to society at large isn't a problem.
Well explained. Although I might disagree about the relative worth of some dogs/humans.