^ I hear ya Indy, and for once, I think I have an answer!
As I understand it, Gibson can't sue Tokai because they wouldn't be able to bring the case in a US court, as Tokai is a Japanese owned and based company, and therefore not 'sueable' (not sure if that's a real word, but bear with me) under US copyright or trademark infringement law.
Also, I believe Gibson no longer see Tokai's guitars as direct competitors to their own product.
PRS on the other hand, are an American owned and based firm, and in direct price range competition, so Gibson were free to sue the cr@p out of them.
That's my understanding of why. Hopefully it's not too far wide of the mark.