I'll go one step farther. I can't really even believe people can just give up their pets to a shelter or to adoption. I mean, I guess if the situation were dire, I could understand. I've heard of tons of situations where people are just like, "we have to give up our dog because we've got a baby on the way" or "we're moving to a place that doesn't allow dogs." Would you consider moving to a place that didn't allow your child? Do you even really care about your pet? I had a pet and looked for apartments, if they didn't allow dogs, I kept looking. Not having my dog with me is not an option.
As for how people can hurt animals, I'll never understand. I guess that's a bit hypocritical coming from a meat eater but ideally even if animals are used for livestock, I want them to have good lives and humane deaths. I'll never understand why people want to torture an innocent animal, but I guess some people are just sick in the head.