Almost as a rule, film novelizations allow creativity within a defined set of boundaries. They are almost always read by people who will or have watched the film, who will have an expectation that what they see on screen will be mirrored greatly in the book. They simply want to see MORE. I don't think writing one is inherently more difficult to write or requires more or less creativity to write than another, it's kind of an apples to oranges comparison. I'm sure if you talked to enough writers of screenplays and novels you'd find examples of both. Filling in the blanks and back-story requires imagination, so does the story reduction and rearranging and wholesale re-imagining that usually occurs when making a movie based on a book.
But you're right that there are a whole lot of movies based on books that are very loosely based on the original material. You'll never see a novel adaptation of a movie have a change in setting or era, leave out major character or completely change their roles... imagine if the novelization of ST3 didn't kill off Kirk's son or blow up the Enterprise.
