As bad as it seems, I think the staff actually have to be "unfair". Art is a very subjective area, and it gets even worse when we venture into the legal gray area of fanart. There are no black and white rules regarding fanart, and I think it will be very hard to make them because every piece of art is different. Everything is basically left up to the admin's judgement. Artists draw in different styles, and admins may interpret styles differently. It's just the way things work.
I just looove hypothetical situations:
Say buttons are a big deal and the dealers might complain about them being sold in the artist alley. Staff then have to make sure that buttons are unique enough so that they do not compete with the merchandise in the dealer's hall.
Artist A and artist B both draw buttons of Naruto. Artist A does a lot of work and doesn't steal any existing work. Her style of drawing makes the Naruto on the button look similar to the actual manga. Artist B puts in the same amount of effort, but draws a chibi Naruto instead; the art on her button looks nothing like the art on the existing merchandise.
The staff review the art of artist A and artist B. They tell artist A that she can't sell her Naruto button, but they let artist B sell hers. Both of them drew non-stolen Naruto buttons, but only one of them was allowed to sell hers, simply because of a difference in style. It's these subjective judgements that cause the strange variations in rules; it's a case-by-case thing.
Art can't really be judged objectively because each work is unique and each artist has a different style. to throw them all to the same category would almost be an insult to the artists. We need to acknowledge the uniqueness of each artist, and, in order to do that, the rules must be interpreted somewhat subjectively. We need to take the good along with the bad, I think.