Because they want the cat to be able to go outside without getting eaten by coyotes or hit by cars.
gandalf24-28I hate to be disagreeable, but why would anyone put a lead on a cat? Because they still see cats as human accessories, and not as sentient creatures with their own intrinsic value, and the same fundamental drive to live by their own leave as we do. Lift the veil. Be awed by their true beauty.