I wonder if it's a carryover of an old regulation that used to make sense. Modern all-season tires are better in snow than the best winter tires were several decades ago.
Also, you need studs or chains to get traction on ice. The difference between a winter tire and a summer tire is the temperature range where the rubber stays flexible. When the rubber gets hard, it will keep its shape instead of complying with the surface of the road, so it loses traction quicker. Ice is flat, so there's no difference between tire types, and there's nothing to grip on to.
Nordic studless winter tires (different from Central European winter tires so also probably different from whatever you get in the US) do give some grip on ice, while all-seasons can be nearly as bad as summer tires. https://link.springer.com/chapter/10.1007/978-3-031-66968-2_...
The government has done ongoing research on these subjects and the regulations do get renewed (e.g. some properly rated all-seasons are now allowed)
Piggdekk in Norway are equivalent to North American studded tires. When I lived in the northern parts of the U.S., I had a set of these for times around freezing rains.
Beyond the questions of winter weather properties, there are adjacent tradeoffs between the tire types (outside of studded):
1. Fuel economy
2. Noise
3. Degree of particulate pollution emission
I'm sure that the all-season tires probably have some negative tradeoffs in these regards to, which yields a choose the most optimal product for the time of year. All-season tires to me seem like a convenience food for places where the weather can be legitimately bad.
One other difference that is hard to articulate to North American drivers with respect to understanding Scandinavia and roads: there are places where snow and ice will literally not be removed (maybe not even removeable) from the road when plowed (I presume until spring melt). It just becomes a thick ice pack over the course of weeks. I never encountered any roads in my life (including Northern Minnesota) that were this inclement. North American roads tend to be cleared (plowing or melting) to asphalt or pavement.
All-season tires aren't simply a matter of convenience, they offer a safety benefit. If you aren't driving at normal highway speeds, even if it's the dead of winter and the air is below freezing, your tires will heat up and the winter tires won't have as much traction. The disadvantage on dry roads can be several times what the advantage was on contaminated roads, including during the winter.
I've also seen snow tires perform worse than all-season tires, e.g: https://www.tyrereviews.com/Tyre/Michelin/CrossClimate-2.htm vs https://www.tyrereviews.com/Tyre/Barum/Polaris-5.htm
If tires complying with the standards overlap, then the standards are meaningless. When there's requirements for snow tires, but not for X brand or model of snow tire, than it's not doing any good. That's why it's important to have a snow rating that can apply to tires of any type, and if it meets that rating, regardless of the rating for dry warm weather, than it should be good to go, otherwise not.