I don't either, but I believe as humans we have a God (or nature if you prefer) given responsibility not to mistreat them. It is one thing to shoot a deer that you will butcher and eat. It is another to inflict as much pain as possible on a deer and then walk away.
That is one of the many factors that separate us fromall other animals.
I agree entirely, but I would argue that we have many moral obligations (God-given responsibilities) that have no place in the legislation of a free society. For example, I think lying is a very immoral activity, but I do not think it should be criminally punishable (unless it's something like false advertising or breach of contract, where there is measurable harm to other people).