It seems like curse is used as a synonym for affliction, malady, or sickness. The shapeshifting itself is the problem. How it is contagious we don't yet know. So its possible this kind of shapeshifting is radically different than the ones we normally see from druids. Maybe a punishment for their avarice, maybe a failure in their experimentation.
I'm still betting Micheal J Fox traveled back from the future to tamper with those druids, causing them to become teen wolves.