I am thinking it is wrong to assume fan fic's are a writers lazy way out of writing their own piece; the imagination and inspiration involved is just as essential for writing both - the context of a said fan fic is just an idea already formed, just by someone else, but whatever is written following the writers interpretation is purely from the writer and original. Therefore, thought out and a great way to get into writing. It can make for good practice.
Writing is serious business and great fun. No matter what.