I know what you mean, but animal derivatives, though not terrifically high quality protein are regulated by law. If you think about it, in the wild an animal (certainly my cat!) will generally eat 99% of what they kill, and that includes some of what we might think of as the less desirable cuts of meat!

As I see it, ingredients such as this fulfill two purposes. 1) they ensure efficient use of the food chain, in that because we are fussy about what we eat there's going to be a lot left from the slaughtering process, and if this can be a food source rather than dumped in landfill that is basically good, and green!
2) It's a cheap source of protein for those who can't afford posh nosh for their pets. It's the pet equivalent of us buying Tesco Saverburgers rather than 100% Aberdeen Angus Quarter Pounders!

I use a pet food where I know what's in it protein-wise, and choose one without the artificial antioxidants (in my case Whites Premium) but it is a bit more expensive than Dr Johns, and if I was strapped for cash I might have to go down that route!