Here is a wiki source (insert error bars here) discussion of his stance on his work being officially licensed. He thought that use of his work outside of a comic strip would cheapen the value of the strip itself. This was frusterating as a child (who wouldn't want a fucking Hobbes plushy) but now later I can see that it was at the very least a very defensible choice. Compare how people feel about C&H vs something that was commercialized to death like Garfield. Anyway, hope it's useful.