Part of it is likely because Norway has done a lot to encourage the use of EVs. I wouldn't be surprised if this were part of this system. There have been things like tax breaks, not paying for tolls, and allowing people to drive in taxi/bus lanes. I'm not entirely sure about the extent: I haven't driven since I moved to Norway. This is all in addition to building out infrastructure - it is far easier to get around with an electric car now than it was when I moved 10 years ago.

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).