The thing is, Lucas always forbade Expanded Universe authors from touching subjects like the Clone Wars, and Luke & Leia's mother, because he wanted to make that material himself one day.
He never set up such a prohibition for what came after Return of the Jedi, in fact he worked with some of the later novel authors to eliminate things he objected to. In Zahn's famous Thrawn Trilogy, the Dark Jedi Master was originally going to be an evil clone of Obi-Wan Kenobi, until Lucas nixed the idea and Joruus C'boath was invented, while he adopted the name Coruscant for the Republic/Imperial capitol world from Zahn's novels (replacing his original Jhantor idea which was suppose to be a homage to Asimov's Trantor from the Foundation series).
Lucas certainly could have restricted authors from writing definiative novels set in the future if he wanted, and asked authors to only write side-stories set during the events of the movies or shortly before, like the Han Solo and Lando Calrissian Trilogies.
He may well have had vague ideas for a third trilogy at one point (of course, at one point in early development the hero was Annakin Starkiller, everybody carried lightsabers and Jedi were called Bendu, so things have changed since the early drafts), and realized that they just wouldn't work out right, and he was more interested in making the prequels so he let the "3rd Trilogy" idea die and focused on the backstory while he let authors play in the future (with him quietly looking on and occasionally nixing things he really didn't agree with).
