What's the legal theory behind "cruelty to animals" law?

Yes, we definitely seem to have a double standard in this area of law, just as in many other areas of law as well. The reason is that our laws were not written all at once. They evolved over time, and were modified by many different people, who were only looking at a particular law rather than the entire body of law on a subject. For example, the people who wrote the law on how slaughterhouses operate may not have even cared about circus animals.