There's all sorts of documentation on the fact that PETA collects every pet they can lay hands on, not just the "unadoptables". This whole story came to light because they scammed someone into placing a nice pet, and that person then followed up...and discovered a whole bunch of nice pets being solicited for adoption then killed, rather than being placed. PETA actually went to pet owners and veterinarians and talked them into handing over their pets -- these weren't just strays and turn-ins.
What's driving them to kill these nice pets instead of rehome them? As someone above pointed out, PETA believes animals are better off dead than associating with man. They don't love animals, they hate people.
They're like the serial murderer who believes he's sending his victims "to a better place".
"Animal rights" isn't about rights for animals. It's about making sure humans have no rights over animals. If you define an animal as equal in rights to a person, then owning an animal is slavery, and the animal has the right to sue you in court (albeit represented by a human...selected by an AR interest). That makes owning an animal too dangerous from a legal standpoint, and THAT is the goal of all these attempts to achieve legal "personhood" for animals.
But for the benefit of animals? Not hardly.
Incidentally HSUS is just PETA in a nice suit. Same beliefs in a more socially-acceptable wrapper and armed with better lobbyists.