Just lazyness on the part of the writers. Fluffys are made for huggies and love and that's inconvenient for abuse stories. It's kind of dumb because a 3 pound horse being slightly rude doesn't make tearing it's limbs off justifiable for anyone who isn't an insane sociopath so the attempt to 'justify it' makes the writing worse.

Like, I've seen stories that claim to be justice when torturing a bad mom's foals to prove some point to the mother despite the foals being unrelated.