The courts can and do, particularly when a particular crime specifies that you violate a specific placed prohibition. In Arrons case (without knowing precisely what went on), it may very well be possible to argue that he thought the banning of the mac address was e.g. a warning, an automated trip/response that the administration may not agree with in his deserving case, related to a period of overload on the system (i.e. temporary ban) etc. And the court would need to believe "beyond reasonable doubt" that none of those were the case. The courts (usually) take a common sense approach like you suggest when your argument of "duuuhhhhhh, they didn't explicitly tell me not to do that" has very little to back it (i.e. no plausible explanation of why you thought it necessary for them to explicitly say it). But backed by a reasonable, or at least feasible, alternative, the courts often side with the defendant.