Because those people have a mental illness that needs to be treated!
My guess is the person has been abused themselves and it makes them feel powerful to have that power over something else. Like if a dad beats his son for misbehaving, then the son turns around and beats their dog for not minding. For whatever reason, some people find abuse to be how affection/love is shown because that's what they learned as a child from their abusive parents.
I'm a zookeeper and I've been bitten by a dog, scratched by a primate, horned by a wart hog, boxed by a kangaroo, etc. ...and I would NEVER consider abusing them for their actions. They're animals and were all acting instinctively. To think that a human should have full command/power of a animal to a point of abusing them is the thought process of a person with a mental illness. I've included some websites for you to check out.
Courtney recommends the following next steps: