If you get the chance to ask, I'd be interested in knowing how involved Lucas was.
I've read interviews with Brian Daley (NPR Star Wars, Han Solo Adventures), Rob MacGregor (6 Indiana Jones novels), & Max McCoy (4 Indiana Jones novels). Each has said Lucas was very hands-off, with just a few rules ahead of time for the books and no involvement with the NPR work.
MacGregor & McCoy mention he told them no Marion and no time-travel, and gave them an era they were to place the stories in (pre-Raiders). Other than that they were on their own. Daley said Lucas told him for the Solo adventures - no Force, no Vader, no TIE fighters. Daley said Lucas was for all intents & purposes completely uninvolved with the NPR radio version of Star Wars.
I find it interesting that, considering how overbearing Lucas is on the Star Wars films, he gives such free reign to people with regards to writing stories containing characters he's created. There are literally hours of non-film story in the NPR Star Wars, and the Indiana Jones novels are completely outside of the four films.
I suspect that the making-off books are much more closely monitored because it's personal image vs fiction.