Eicca for one is a savvy businessman, so I'd imagine he plays a HUGE part in the decision-making. And I'm sure the rest of the band plays a large part as well. Popular radio just doesn't play songs without vocals (when was the last time you heard one in heavy rotation that wasn't techno?) so Apo's doing what they can to get some airtime. Radio airtime is where record sales come from, concert sales come from, etc. and they're in this to get paid after all, otherwise they'd still be playing heavy metal clubs in Finland on the weekend.
I do agree with you guys though that there is a potential for new fans to be disappointed. Apo's done extremely well though in making sure a wide variety of tracks are available to listen to on their Myspace and website. The whole point of the singles is to drum up interest and draw people into researching the band more. I think in the day of the Internet, the number of people who hear one or two songs and buy the entire album is small.