Not a serious car guy, let alone a tire guy, so didn't read the entire thread, sorry if I missed the answer.
But here's the answer, anyway: http://www.epa.gov/climatechange/pdfs/climateindicators-full-2012.pdf
It's actually using a lot of data that don't even include the current drought out west. But if you start reading along about Pg 53, you'll see that average NA snowfall has declined significantly, and that the ratio of snow to rain is dramatically smaller. (And other data sources show that length of snowfall season has shortened by nearly a week.) You'll also see that the biggest decrements are in the Pacific Northwest, down to middle Calif, and in the Middle Atlantic region. Both areas have plenty of mountains and hills, and plenty of people. The biggest increments, not nearly as dramatic, are in relatively flat areas of the northern U.S. and in comparatively sparsely inhabited parts of the SW. So I'd guess that folks in the northern U.S. are still buying their snow tires, but people in the NW and Mid-Atlantic, not so much. Also keep in mind that more and more of us live in cities, where real snow tires are not as vital as they used to be; better snow removal, more salt, + lower snowfall + more rain. I'm not going to slap on snow tires every weekend I drive to a ski resort, any more than someone in San Francisco or Seattle will. My aggressive all weather truck tires and NH and VT's road departments work fine as long as I'm a sane driver.
Now here's a counter-puzzler: If you didn't buy snow tires where conditions no longer demanded them, how much carbon would you save?