I have no doubt of this Train. Movie ratings are made by the Classification and Ratings Administration (CARA), of which all persons of that panel are hired by, and placed in, the position as a panel member by the CEO of the Motion Picture Association of America.
And while the ratings are a voluntary rating system, you just can't get a movie in a theatre without getting it rated, which slips the next little slimy hand for a payout -- NATO. NATO stands for the National Association of Theatre Owners.
Together, MPAA and NATO pretty much call the shots, but understand that MPAA is merely a front. At present, MPAA is spending an inordinate amount of money trying to battle piracy, so donations to such a "worthy cause," by MPAA members (the movie studios, and thus the producing companies), would no doubt influence the ratings applied to movies. I mean, if you don't satisfy the members of MPAA, they'll stop being members. MPAA doesn't dictate, it serves. Think of the movie producing companies as the restaurant owner, MPAA as the waiter, and the ratings as "what's on the menu."