Why is it so cheap to park your car and not others?
This usually means you can't stay for a particularly long time.
Here is a map of all charging points provided by the municipality around Oslo: https://experience.arcgis.com/experience/2053e5d9f24c48c487e...
This program isn't the same throughout Norway, though. In Trondheim, it looks like you pay for parking and then you pay for the charging separately so you really aren't parking for free. (Private parking might vary, and sometimes city parking is free).