I do not know the Swedish law either, but let's take it to an extreme:
If someone were to run a site called Assassin Bay, where clients could post .target files specifying jobs and hit men could accept those jobs, I'm sure that the site would be taken down within hours and the site owner would go to prison. Even in Sweden.
EDIT: To allow for the possibility that the Assassin Bay also has legal traffic, replace .target with .ad files. Then assume generously that advertisements for legal jobs make for 80% of the traffic and 20% are hits. The argument I made would still hold.
I think a much better analogy would be if someone were to run a site called "Craig's List," and had a section devoted to 'personals.' Would the webmaster be _legally_ responsible for those using that service for prostitution?
(again, not the site's moral responsibility to itself [craigslist cracks down on this sort of thing], but to the point of being able to arrest and imprison craigslist webmasters.)
If a site has been running for several years and a substantial portion of the ads are illegal and during all that time the site admins have repeatedly refused to remove illegal ads, then yes, the site admins have some responsibility.
If someone were to run a site called Assassin Bay, where clients could post .target files specifying jobs and hit men could accept those jobs, I'm sure that the site would be taken down within hours and the site owner would go to prison. Even in Sweden.
EDIT: To allow for the possibility that the Assassin Bay also has legal traffic, replace .target with .ad files. Then assume generously that advertisements for legal jobs make for 80% of the traffic and 20% are hits. The argument I made would still hold.