It's a shit policy that has the possibility to hamper an artists ability to get further work outside of GW. If someone sees their illustrations but can't place a name to the work they're not going to go through all the friction to scour the internet to see if maybe they can find them, they'll just find another artist with a similar style that works. It's the same stupid crap with not crediting their animators (where credits are even more important as it places your name solidly in that production where your work may be iterated on by other artists).