The catch-all law...trespass. Unauthorized wifi and wireless access
when in doubt, when you can't find a law that covers unauthorized Internet activity, everyone turns to trespass. It was the law used by AOL and others when fighting SPAM, claiming that the SPAMMERS were accessing their networks without authorization, and therefore committing trespass.
Trespass arises under state, rather than federal, law. Here too you will have to identify some kind of damage to the person from whom you are netnapping access. Once again, the larger the files, the more frequent the activity, the more easily your unauthorized access could constitute actionable trespass.
but forewarned is forearamed.