I disagree with the notion that it takes less skill or imagination to work within an established setting (or, more generally, within established parameters) than without. Doing more with less is the foundation of engineering. In the context of an established setting with established characters, to write a good novel you have to:
1. Maintain character integrity
2. Expound or develop characters
3. Make them do something interesting and worthwhile
Now, many may skip part 2, and that is indeed being lazy and unimaginative - but then the writing isn't worth your time beyond entertainment (which is not a terrible end). But this is much more difficult than just making up your own characters, as is evident in the fan fiction domain - the whole trouble with overuse of Mary Sues that people are talking about is because it is too easy and unimaginative to do just that!
Moreover, the same can be said of setting. Being unable to change the elements of the setting at your whim is a challenge, not a crutch. An example in popularized science fiction is that will probably strike a nerve is Star Wars Episode 1. If it was written as a tie-in (using established characters and setting, no less), they simply wouldn't be able to pull the whole midichlorian nonsense out of nowhere; it would be contrary to the established setting, and therefore would either be not acceptable or non-canon.
Not only that, but things like themes and plot are separate from setting and character, meaning you can take the same setting and character and use them to build both a dramatic, meaningful story and one where nothing really happens for no reason.
Long story short, tie-ins are neither more nor less than fully original works. It's easier to be lazy, but in the end it's what you do with them that counts.
Being that I started writing as a fanfic author, it's actually harder to write good fanfic that is true to the vision and characters of the original story- because if you don't do that, if you just lazily slap up tripe and bullhonkers, you will get flamed for it. It's actually harder to write in someone else's vision because it isn't your own- you have to stay within the boundary of the worlds and the characters, you can't just use and distort the characters at your whim. For every difference you show between the characters as portrayed in the original media and your version, you have to show your work. You can't arbitrarily change the story at your whim- you have to stick to what is already there.
For some writers, that's a challenge. for others, a straitjacket. The best fanfic authors tease out stories that are hidden within the canon ones- sticking to the canon story while adding something that you didn't see in the original, but which isn't contradicted by what you saw/read/played.