There is another possible/probable explanation. I worked in insurance a long time ago and it's as devious and dirty as any investment/speculation business. Right now insurance companies are still hurting from the effects of 9/11 and more recently, the economic fallout from the Icelandic volcano. When you add that to the current economic situation it is easier to see why this industry, which operates at the bottom of the trough along with banking/bondholding, is desperate to claw back their losses anyway they can.
To look at what's happening it is necassary to understand that the primary business of insurance companies is investment via speculation on stock markets/armaments (see starting wars/invading third world countries) oil/pharmaceuticals etc, and the way they aquire the capital to do this is by selling insurance policies, much the same as the major supermarkets/bulk/discount retailers. The nett directive is to increase the value of the company shares through the maximum profit/minimum time using whatever means.
Hence the issue of winter tyres commanding a higher insurance premium is actually counter intuitive/productive but it's deemed a viable source of increased revenue. An example of this is where the insurance company ask your occupation. It doesn't matter what you answer, your premium is loaded under the guise of risky profession even if you say " I use public transport Monday to Friday and the car is kept in a secure garage during the week and only used at the weekends", you will still be loaded. This was one of many reasons I was pleased to leave the industry although car indemnity was only one of the areas I worked in, the operational methodoligy behind pensions and investments turned my stomach then and still does now.
