Maybe I missed the point, but I thought that McCartney's point had to do with witnessing the distress (and the killing) of the animals, not with the specific practices.

So I thought that if you're willing to kill the animal yourself, then maybe you're able to condone the slaughterhouses even if you see them through a glass wall

(maybe it will diminish your appetite, though :P - I wouldn't want to see it while eating!).