Here's the thing though, unlicensed products are usually copying official merchandise. Or, are trying to look as official as it possibly can. If it were art, it'd be like someone tracing or recolouring.
When someone draws fanart, so long as they're not tracing, redrawing, or recolouring, the artist is taking their time and effort to create something. Great art is great art, regardless of the content. If you want to argue fanart shouldn't be allowed, I would like to point out that several famous works of classical art can fall into that category.
I buy art that makes me happy, be it original or fanart. Artists should be allowed to create whatever makes them happy.
And of course, with fanart the characters are more reconizeable, and the postive emotions people have attached to them help determine which fanarts people buy. With an original character, there's less of an attachment to people who are not familiar with the character. I'm not going to buy something original, if it doesn't trigger an emotional (be it a happy or sad one) response.
I create my own original stuff, AND fanart. People like what they like, and I'm not going to chew someone out or be offended if they like a fanart peice of mine over something original that I drew. That's just immature.
ALSO, to go off something Arch said, you only really know something is fanart if you know the characters are from something. I've seen plenty and purchased art before that was really fantastic, and found out later it was fanart. And you know? I didn't care -even if I ended up not liking the series - the art was WONDERFUL, and I wanted it.