The way I understand it, a concave back on a weight like that would require a three piece mold rather than the standard 2 piece Do-it Mold. I believe that they've played with this in the past and found the mold difficult to pour consistently due to the added pieces in the mold.
I think most concave worm weights are machined a bit after casting, but I don't know that for sure. It boils down to design and trying to make it functional so that any Joe Schmoe can pour the thing without troubles. I'd never want to see a mold brought to market that will cause anyone an undue head-of-an-ache.