What if the business's purpose is to provide an open suburban hotspot?
Now, if you argue that the judge should follow the letter of the law, and ignore the spirit, remember that the first amendment protects the freedom of "speech" and the "press". It doesn't say anything about websites. Of course, the spirit of the law was clearly anti-censorship and would obviously apply to written texts that haven't physically been through a printing press, but the letter of the law says no such thing.
Alternately, you could stick to the letter of the law and simply redefine the word "speech" to include things obviously not spoken, like websites.
Convincing judges that the spirit of the law does not match the letter, or that the dictionary definition of a word isn't the definition to be used in the case, are ways of hacking the law that have nothing to do with judicial corruption.
It is similar to the case of catching a speeding car but not being able to identify the driver (except speeding isn't a tort). You can't assume the owner is guilty unless the law is drafted to make it so that a car owner is responsible for all authorised use of their vehicle.
Talking about the spirit of the law is considering the law makers intentions and how they would have drafted the details in the current technological background.
Even if you narrow down the guilty party to one of a small group you can't convict the whole group or even a random member of the group on that strength alone. That's not how European or USA law works (though there may be other crime, withholding evidence, harbouring a criminal and such that members of the party would be guilty of).
However, I want to take your speeding car analogy a step further. The police have video of my car parked outside a murder scene during a murder. The murder was performed with my handgun. On the 911 tapes, the victim is shouting that he's being attacked by someone with my name (John Smith). The victim is someone I've previously stated that I intend to kill. The argument that none of those identify me aren't going to form a reasonable doubt unless I can also provide an alibi or implicate a different John Smith.
In the same way, the original poster has publicly stated their intention to start an open network for the specific intent of hiding their downloads. The MPAA has an IP address that shows that a download was made from the account that was purchased by a person intending to hide downloads. If they have server logs showing that other people use that network on a regular basis, they have a decent chance. As it stands, however, it's pushing the bounds of reasonable doubt, not to mention preponderance of the evidence, as needed for a civil case.
You say: "excuses that would easily fool a computer probably won't fool a judge for a second any more than it would fool a guy on the street."
I don't think it comes across as an excuse although prosecution would of course present it that way.
There is certainly instances where someone would provide an open hotspot for neighbors. (My neighbor at one building did this.) I don't think it would be difficult at all for this to be shown by any defense attorney after some research with examples of kind people doing the same. I don't think this is as open and shut as you are presenting it. And that is the job of a defense attorney to try to make things appear possible by presenting evidence of the practice (which doesn't have to be widespread either.)
There are of course ways to address this via legislation--but that will never happen as it goes to the very core of the corporate personhood issue.