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.