I work in the guitar OEM trade. "Ghost building" is not a term we use, so I'm not 100% sure how moucon is using it. When a brand outsources their production, it's called contract manufacturing. As the name would imply, there's a contract in place whereby the contract manufacturer gets paid to build the guitars under a license of the brand. That's how more than 90% of guitars are built these days and it's similar to ghost writing a book. It's 100% legal and there's nothing unethical about it.
However, when a guitar builder puts a well known brand on the headstock of a guitar that was built without the brand's knowledge or control, that's counterfeiting, plain and simple. It's no different than making a fake Rolex. And its no different than stealing music.
It's true that Slash revitalized the Les Paul with a counterfeited guitar and Gibson gained from it (so much that they reissued that guitar themselves). But that's just dumb luck and it doesn't justify counterfeiting. I know for a fact that Fender and Gibson and most of the big brands have very aggressive legal teams that will take action against counterfeiters to the extent the law allows.