They're PARTS of Unseens. Recognizable parts, like Marauder arms or Phoenix Hawk jets. Is it legal? Shady area- and we know where that got us once before.
IWM is likely covering their butts and making sure there's no reason for anyone to send legal ninjas after them, and I for one have a lot of trouble faulting them on that. Never mind that it would be creating individual parts (which sell for, what, two bucks each at the absolute most?) for miniatures they don't even make- which means that's all they're making, a scant few dollars from each molding. How many people have unseens that need new parts? Several, no doubt, but the vast majority of fans- myself included- sure don't.
So, very little money to be made, sculptor has to get paid anyway, potential legal nightmare... yeah, not happening.
*DISCLAIMER: I have no standing with IWM other than frequent customer, and do not know their train of thought- the above is merely my guess on the situation, one that I have high confidence in.*