As far as I know - and remember I live in Switzerland - there is no "legal" requirement for winter tyres.
Instead the requirement of your insurance is that your car is adequately prepared for the prevailing conditions. If not, and you are required to claim, your car may not be covered.
Futhermore if you were in an accident the police may apportion greater blame on you should your car not be adequately prepared - this may mean going from a non-blame to a blame accident.
It is one reason why, after 20cm of snow last night, I am not moving my Alpina today - as it sits of summer tyres