The copyright laws on fan art is pretty clear, but there are some cases where copyright holders may choose to ignore it, especially if the fanart is helping advertise their overall product. Usually, companies will completely ignore freely distributed fanart which is done free of charge. Sometimes fair use may protect fan art from being a copyright infringement, but this would have to be evaluated on a case to case basis. Most fan creations, especially those for profit, do not criticize or parody the source material, so they are not fair use.
Basically, it's the responsibility of the artist. I think CGT's stand on this is you can create "fan art" but it's at your own risk. If a cease and desist letter comes through, then not only will your product get cancelled, but you may end up in court. Just look at what happened with TS and that whole BMW car model issue.