Cameron hardly had that sort of weight back in 1997. Especially when Titanic had gone radically over budget, was backed by two competing studios and was a huge, huge gamble that no one expected to pull off what it did. Kate's nudity didn't warrant the R rating not because of the politics of the MPAA (which are messed up) but because of the context they were shown in. They were shown in an artistic context and not for titillation. If she had shown her boobs in a sex scene the movie would have been rated R. But it was an "art" scene so they could be shown. You can even show breasts on broadcast TV in the right (though narrow) contexts. As what happens when Roots is shown on TV, an episode of the TV series "Chicago Hope" had shown the results of a woman's breast reconstruction surgery and the various incarnations of CSI have shown topless female corpses in autopsy scenes. Never the less, the MPAA and the ratings system in America can be a bit messed up when it comes to sex and violence (the former being a taboo the latter being okay) and in showing preferences depending on the producers/director of the movie.