If you found this sign in a parking lot, as clarified in the comments, I would (as a Norwegian) interpret the sign to only refer to camping with caravans and motorhomes and that it in a rather convoluting way is trying to say that it is not allowed to stay for more than one night with a vehicle. The Norwegian word 'camping' is not identical in meaning and coverage to the English word and in a Norwegian context usually only used to describe staying at a commercial camping site or living in a caravan (campingvogn) or a motorhome (campingbil or bobil). Staying in a tent outside a commercial camping site, would usually be referred to as 'telting' or in English 'tenting'. The English translation on the sign is simply sloppy and makes the impression that the restriction is much wider than actually ment.
As I already commented on lambshaanxy's answer, the freedom and right to roam is anchored in Norwegian law and the municipality or even a private land owner has very restricted legal possibilities to deny tenting on a property. Since Femsjøen is a source of drinking water, the municipality would in this case actually have the possibility to restrict access and conduct around the lake, but there does not seem to be any.
Staying in a caravan or motorhome is not covered by the freedom to roam, but may be restricted by municipal parking regulations. It is not uncommon that staying in a parking lot or at a rest stop is limited to one night.
Other municipalities manage to inform about such restrictions in a much clearer way than the sign you have found and posted in your question. The municipality of Nesodden uses signs like these, where for some reason only parts of the text is translated to English. The pictograms should perhaps already make it clear that the camping restriction only applies to caravans and motorhomes, but the Norwegian text makes it absolutely clear:
Tenting and other stays at the property are not allowed for more than
two days.
