Copyright law generally exempts satire, which keeps many comedians, and "South Park", from renting sleep space in civil courts.

Fan fiction is free advertizing. It's so often equal in quality to the lower end of the original works, and sometimes even superior to them in the case of outstandingly talented fans, that the copyright owner would be foolish to restrict their production. Sometimes they provide fodder for new life to the franchise.

I daresay Lucas doesn't even mind people making money off of fanfic as long as he gets a piece of the action. Prosecuting your admirers seldom wins you popularity, but even fans often understand prosecuting serious piracy.