If I had to guess, the reason why block heaters aren't mandatory is because they are rarely necessary, especially with petrol engines. Helpful perhaps, but not necessary. (Yet I do have a petrol-operated heater to help reduce harmful emissions and possibly engine stress -- if only I remembered to use it those rare occasions I store the car in the cold. I've yet to come across a situation where a car wouldn't start because of temperature here.)
These areas, in Siberia and northern NA are much colder at times, even when they aren't within the Arctic Circle, so their situation might be considerably different.
Yes, I think it has to do with most of the Scandinavian population being in areas that just don't get cold enough to need a block heater. Many people have one anyway, but a modern car doesn't really need a heater until it gets below about -20 C. And the biggest Scandinavian cities (Stockholm, Copenhagen, Gothenburg, Oslo, Aarhus, Bergen, Malmö) rarely get that cold. A typical winter day in those cities is gray, hovering within a few degrees of freezing, and covered in a light mist of rain or sleet.
Now if you live in Kiruna or Luleå or so, things are a bit different...
You're right. I was a bit sloppy, I'm from Finland which is not geographically Scandinavia, and it's somewhat colder here than in Scandinavian population centers which are south of 60°N. Though the Golf stream helps us too, it's much warmer here than in Alaska at same latitudes.
[edit] It is true that modern cars don't need a block heater to start up successfully, but heating the engine before starting reduces fuel consumption and emissions radically.
These areas, in Siberia and northern NA are much colder at times, even when they aren't within the Arctic Circle, so their situation might be considerably different.