This is more likely a business decision on the part of Kadokawa rather than the author. Since there are figures being released for the Guyver, this copyright protects them from third parties developing unauthorized (pirated) versions of any merchandise they produce. Because if there is no copyright on the IP, you can be assured that a lot of 3rd party will profit from it. At least this way, even if 3rd parties develop any form of merchandise related to the IP, Kadokawa still profits from its licensing.