Good question. The verdict actually states that you normally shouldn't be sentenced to prison for this type of crime. Instead the court motivates the prison sentences with upholding "law-abidingness" as file-sharing has become "a problem for society". Which is a bit scary if you ask me.
> As a citizen of neighbouring Denmark, I find this a truly grim day. How can something like copyright infringement result in a prison term ?
I'd guess it is similar to the US: if you aren't doing it commercially, you can't get prison--it is purely a civil matter. If you are doing it commercially, then it can be a crime punishable by prison.
TPB was/is commercial, making millions a year monetizing copyright infringement via advertising.
And don't even get me started with the whole "A torrent tracker is copyright infringement" argument. Jeez!