Norway has very strict pro-workers laws in general, it's just one facet of them. One Norwegian explained it to me like that: in the late '60 when Norwegian oil industry started developing, workers realized that they can incur great losses on the companies if they organize/unionize and strike together. They used that as a leverage to both change their contracts (to include paid sick leave and such) and also get better working conditions (Norwegian platforms have both better safety and on platform to on land ratio).

And later other trades did the same. Some of the things in contracts trickled down to the law. But still some laws apply only to companies where at least a certain % (is it 50%?) are unionized.

The general picture is more or less like that, but please verify the details.