My courtroom scenario wasn't intended to resemble reality, and I'm surprised you didn't realise that. It's called an illustrative analogy, and was intended to highlight one of the problems facing people who think that this is a straightforward matter.
If you read the terms and conditions on the website in question - and you obviously haven't - you would see that there is indeed agreement to them when someone chooses to enter the site.
There are many examples of successful prosecutions where the person being prosecuted claimed that someone else was using the machine at the time. People have been prosecuted for copyright theft by the music industry, and on more than one occasion the defendant has claimed that another person was responsible for the illegal downloads - one father claimed that his young son was to blame, thinking that the court would find in his favour. It was ruled that he, the person who paid the ISP charges was liable, and he was fined as a consequence.
If something is legal (just) - to use your terminology - it's legal, there aren't degrees of legality. Whether or not the MBS payment demands would be enforced by a court is debatable, but so far as I'm aware it hasn't been tested, so nobody knows.