The Americans have been using Russian gear in Afghanistan since 2002 and have been training on Mi8 series (mtv) in the states for many years, the performance of these types at elevations and temps experienced there and their demonstrated reliability in that theatre (both Iraq and AFG) is the reason. There is no American equivalent. You would need to replace these guys with NH90 to get a similar result at USD18m a shot you can have between four to five used 17's with a proven track record sionce the 60's and no waiting, remember also that the MI8mtv is the most battle proven helicopter in the world today. Its a common sense approach and not the first time the Americans use Russian aircraft, we have been supporting them for years in places like Afghanistan/Iraq/Columbia etc etc etc and continue to do so, it make more sense for them to own the machines than lease them continuously.
Cheers