While playing the Empire campaign in Star Wars: Empire at War, I've noticed that the Rebels' fleets sometimes ignore my space defenses if I only have a space station and no ships.
More specifically, I remember the Rebels were able to move some ground units to Endor, and attack my ground forces there while ignoring my space station. When playing the Rebel campaign, I was able to do the same thing to Imperial-controlled Mon Calamari.
However, I was unable to replicate this behavior for other planets, so I think it might have something to do with whether the local population helps you (both Ewoks and the locals on Mon Calamari help the Rebels).
Could anyone clarify how this works?