Fair point. Perhaps a cage around the sand pit would work. However maybe the fundamental moral failing or injustice is the domestication of animals. Maybe "owning" a pet is immoral in itself ? It violates the golden rule "do unto others..." unless that is we ourselves would desire to be owned by another being. Now if on the otherhand a stray cat not owned by anyone did his thing in the sand pit then nothing immoral is taking place. Ownership entails responsibility.