Dude, this is just what Greg Cox told me. A screenplay has 100 pages on dialogue only, and the writer has to make everything up all by himself, and eventually diverges from the movie. Cameron doesn't want a novel to Avatar to be like that, so he's going to write it himself.
It's worth remembering, though, that the final manuscript has to be approved by someone at the studio, who has, hopefully, seen the actual movie. This keeps things from diverging too much . . . .
It's not uncommon to do last-minute rewrites to make the book more like the movie--especially if there were last-minute changes to the script.
"Bad news, Greg. They shot a new ending. How fast can you rewrite the last fifty pages?"