Jeff Kouba at Peace Like a River takes a look at the report of Iran training Chechen terrorists in Qom with a post called "Doing deals with the devil is risky business". (If you haven't yet, spend a few reading at Jeff's place. He does some good thinking.)
In this instance, he chooses to look at this particular issue from the perspective of Iranian motivations and offers several good possibilities of why Iran would do this.
He has left the true reason off the list, however: Because that's simply what the premiere State Sponsor of Terrorism does.
While he rightly reminds that Iran has been doing the dance and playing the game, it's really as simple as that.