The difference between fan art and bootleggin boils down to the money. Hell, I created an Oldboy design in the past but I've never actually sold it. And neither would I ever intend to. It's a tricky subject and the fact is no one will give that much of a damn. Once you associate/incorporate an established creative product into your work you're going to get other fans of the subject hooked to view the work. I think that's legit to show what you can do and to show your love for a particular thing, but selling it, for me at least, is a no go.
I'm sure guys like Olly Moss in the past have created pieces depicting commercially successful films/figures without permission, and I also think as a result this has garnered interest from the actual owners of the IP to commission them for official work. It sucks for the guys that try to go through the right channels and props to them for doing so! I cant see it stemming the wave of new guys jumping on the bandwagon trying to make a quick buck though.