A lot of people, including me, agree with you. The "mystery" is the ratings board, no doubt about it...last I read about this issue, this film's odd bypass of the NC-17, was that the ratings system itself was denigrated and thus, this film got a pass (conveniently, is the point, the ratings system was called into question where this film was/is concerned, the system alleged to be nonsensical and not meaningful, and so, hey, it doesn't apply to this film!). That's my summation of an article I read in VARIETY not too long ago, trade paper for the film and television industry.
Hey Annie Proulx,
Drop your holster
forget about the pollsters
Become a seeker
Seek both night and day
Ask the Lord to help you
And he will show you the way
Go tell it on the mountain
Over the hills and everywhere
raise your hands toward the heavens
Annie, go tell it on the real mountain
Jesus Christ is born