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.

If you read Swedish, this conclusion is on the top of page 47 in the verdict @ http://fildelning.se/wp-content/uploads/2010/11/TPB.pdf