The facts of the actual case make it much easier to understand the problem, but how is that situation actually different? There are still multiple people it could have been, no evidence is presented to distinguish between them at all and it's an obvious injustice to punish one person when it was another who did it.
You don't, that's the point; claiming "the criminals had mail sent to 1342 Grass Lane, therefore the owner of 1342 Grass Lane is the criminal" is patently ridiculous, and it's good that the court realizes that the same applies (if anything, even more so) to IP addresses.
