I think the most important reason is bandwidth, they don't want 24/7 bandwidth hogs, and torrent users usually fall in that category. VPN are meant to use for web browsing and occasional file download.
A VPN provider that allow torrenting, would either charge a much higher fee, or go out of business very soon.
VPN =/= VPS
And btw, mullvad.net allows everything, there's no transfer limits, and it's only $5/month. And ou can pay with Bitcoins of course. I can't imagine better service.