Then you haven't been following the news over the last few months. Lots of folk metal bands have been getting signed by important labels in heavy metal. Its a trend.
My assertion of their cover of Rasputin may be off, but I've noticed more signing of these bands lately, and I would not be surprised if the success Turisas has had with it is what caused it. Its also a normal thing to occur in the music industry. One band of one type finds some decent success, and then labels have to jump on board to get their bands of that style.