How could Jim Morrison do a live concert in 1991 when he died in 1971?

In 1991, Ray Manzarek (Drummer of The Doors) and Robby Krieger (Guitar Player of the Doors) played a concert with Ian Astbury replacing Jim Morrison and they calle themselves The Doors, but of course, Jim wasn't there. Neither was John Densmore, the drummer, which led to a lawsuit in 2004 over the licensing of the band name "The Doors"