First, to clear up a point people seem confused about: a misspelling of an author's name makes a mini of the same type as any other misspelling in the continuum. J.J.R Tolken, Tolkein, Tokein, and so forth are all mini-Balrogs.

As to why they aren't adoptable, we're talking about the name of a real person here, not a fictional character. Not only that, but a real person regarded as the creator of a universe. Both from an out-of-universe and an in-universe perspective, an author mini has a greater status than others of its type—one that should be treated with respect, not made into a commodity for a few to possess and others to envy. It would simply be in poor taste.

Why hasn't this been spelled out up till now? It was just understood. But I suppose that's because we all used to come from more or less the same fannish background, and now we don't.

~Neshomeh, sighing in grouchy old person.

I'm not an expert on PPC lore, but when it comes to minis of particular canons, a general rule of thumb is to avoid coming up with mini types that are based off human or humanoid characters, unless there's absolutely nothing else you can use. Humanoid minis tend to look like toddlers, but minis are seen as pets, and combining those two to keep a human mini is a bit... weird.