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.

"you should know" and "you did know beyond reasonable doubt" are two different things, particularly when it comes to prosecution.

Running open networks without explicitly communicating and agreeing terms of use with your users really does muddy the water in these cases.

"His MAC address, which is meant to be specific to a machine"

The article (which I tend to agree with) tends to argue that a MAC address is exactly NOT that, especially in a technical sense (thus a technical user may indeed have more reason for the intent not to be "clear").

I think a better description in plain english is : A MAC address is an address that a particular interface on a particular machine asks to be identified by in a particular session.

I.E. it's not an identity of a machine, its an identity of an interface. Its not an identity assigned by the network, its an identity offered by the machine/interface itself. Its not guaranteed to be unique, or stable beyond a session.

If it was meant to be a specific identifier for a machine, we would have many technical problems on the "legitimate" side of things, think multiple network cards and virtualised machines.

There was no agreement between Arron and MIT that arron would use a particular MAC address as an indentifier on their network. Its a downside of running an open network.

I don't like the bouncer analogy. The network people surely know that the MAC filter is ineffective and no bouncer would be fooled by a fake mustache.

I agree that it is clear enough that Aaron Swartz was intentionally circumventing their attempts to keep him off the network. Where I have trouble is that any notional security mechanism is apparently enough to make the circumvention a serious crime.

I suppose where I am going is that severe penalties should be for circumventing security features, and easily altered implementation details of network hardware should not qualify as security features.

No it's not. What if they blocked the MAC because they detected botnet activity? There is absolutely no desire in this case to ban the person, or even to ban the entire computer if it has multiple OSs.

There is really no way to tell just from a MAC ban what the intent is.