How would this be cheating? If the Jets can't control access to their playbooks then it's their own fault.
if you knowingly acquire stolen property, it's still a crime even though you didn't steal it.
getting a copy of a playbook, which you know you shouldn't have a copy of and you know the owner of would not let you have a copy of, is still cheating even if you didn't take the original. yes, the Jets have responsibility to keep their house in order, but that doesn't absolve the Pats from owning a playbook that they know they shouldn't have. If I leave my door open, and you come in and steal my TV, you are still a thief.