Search:

Found this. Except for what appears to be a clearly identified opinon it seems pretty good.
http://www.dove.org/news/uplink/hu0504.htm

Excerpts:

Still sounds like a private orginzation (MPAA perhaps)

You are correct in what I meant by enforceable.

Actually only R and NC-17 are the only "barring" ratings.

And what is NATO is this context?

Edit: Ah found it. Yeah its a bad acronym

Are the movie ratings even legally enforceable? They are suppose to be a voluntary thing to keep the government regulations out of it.