@joylyn: like kayura said, technically, when you scan a DVD cover/ artbook, it's still copyright infringement. However, my view of it is that if it's printed (in the case of fanart from pixiv or somewhere, an artbook for example), then at least the original artist intended it to be seen and distributed through printed means (and it is sold, thus they made money off it), whereas if they upload the digital version on say pixiv (or devART for example), they only want to display it and not have it distributed. It's a fine line, and no, scanning it and uploading it doesn't make it any more or less legal (because technically, only the printing company has the rights to distribute really) but I think it's more of a morality/politeness thing really.
As for can you upload the digital version if you have the print version and don't want to scan it etc, I'm not a scan mod, but I think it's just easier to scan the artbook if you can to make organization easier. Meaning that we can tell it's actually from an artbook (ie the image has been published), and exactly where it's from, as Japanese artists don't usually sign their work.
@davidfoxwv: while I see your point about the "ivory tower", I don't really see how it has to do with this decision, considering it was pixiv who has asked us to take down stuff from their site, and I don't see why we would say no to their request.
But in general, yes MT has a "narrow focus" as you put it; it's not an imageboard like Danbooru is, therefore, it's not going to have the same functions and rules as an imageboard. So people who have come looking for an imageboard are in the wrong place, and are probably better off at Zerochan or Danbooru. I don't think MT's goal was ever "keep everyone who comes happy", while that part is good to have and important, we do have to have rules, just like everywhere else, to keep order. These "dissatisfied customers" you seem to talk about are probably in the wrong shop.