I don't think it's so difficult to understand.

These are people who have absolutely no emotional ties to the animals they are being cruel to. Pet owners are the only ones that develop that kind of feeling towards animals and keeping pets is a luxury most people on this planet can't afford. If they keep animals, it's usually so that they can feed their family and you generally try to avoid getting too close to something you are going to have to kill, gut and skin. Anyway, these people feel about as guilty about cutting off living frogs' legs as any one of us would feel about ripping a living vegetable out of the ground and throwing away everything except the root.

Note that I said "understand" in the beginning. Understanding someones actions and approving them is not the same thing. I wouldn't do what they do, but then again, I'm not in a situation that requires me to do it.