sanmac - // … but for adults to be "afraid" of them is a little buffoonish. //
I suggest then that you don't suffer from any phobia yourself?
Any adult suffering from any phobia is entirely aware of the irrationality of his or her fear, but a phobia works on a sub-conscious level and appeals to the rational mind go unheeded.
I would imagine a fear of clowns may date back to a circus visit as a child, when an encounter could be frightening for a small child.
As for Mr King's wonderful creation, and the entire 'bad clown' nonsense that has piggy-backed onto it, it is his fiction, but it has had that unpleasant consequence as a result - a testament to the reach of his appeal and his skill as a writer.
I believe that any phobia sufferer deserves the respect of a little understanding, even if you are not a sufferer personally.