Ah, the Saint Hannes cross, or sankthanskors in Sweden, or hannunvaakuna in Finland. It's not so much related to campgrounds, but to mark sightseeing spots in general.
The use discussed here is established from the 1950s onwards, first suggested by a local history society in Finland.
E.g. in Norway the sign is specifically described in regulations as referring to a "severdighet", literally something like "a seeworthy thing" but generally translated to "attraction". It's specifically regulated to mean that [1], rather than fornminne/ancient monument.
In Norway, you can for example find it used for the Holmenkollen ski jump, which is hardly an ancient monument [2].
[1] https://lovdata.no/dokument/SF/forskrift/2005-10-07-1219/KAP...
[2] https://www.google.com/maps/@59.9612567,10.6669888,3a,75y,10...
An athletic facility that's been going since the late 1800s and has a dedicated museum and is of distinct local cultural importance kind of fits the "historical interest" part pretty neatly.
As your lovdata-link shows, there are six different signs for severdigheter, not just one, and in the local parlance severdighet typically refers to things like historiske steder, monument and so on. Unless something has a bit of history it's unlikely to get one of those signs put up.
The other 5 are for narrower, more specific use, and demonstrate quite clearly that the word "severdighet" in Norwegian has nothing specifically to do with historical interest. When you then try to insist that the general, catch-all sign does it feels intentionally obtuse.
To make this clear, here's an example of a sign to INSPIRIA Science Center[1], built in 2011.
https://g.acdn.no/obscura/API/dynamic/r1/ece5/tr_1200_1200_s...