Anyway I had always assumed that Art Of War had the Berserk license for statues, action figures, mini figures and Jewelry. While other companies had things like keychains and block figures. The Banpresto PVC statue just made me curios.
It's probably just an arrangement between the two companies (or directly with Hakusensha, not sure if AOW's license has some sort of exclusivity clause, that's what DirectDK was referring to I think).
Headlong, Dotou No Tonosama, Mimyan and the other garage kits sculptors have to get special temporary licenses to sell their creations, and they can do it only during "Hobby conventions" (they're being resold later on Yahoo Japan auctions). So I guess a simple business agreement would be the likeliest possibility.
As for the details of the respective licenses, I'm not sure they've been publicly announced.