By many people's shorthand view of copyright, it seems obvious that Glee (ultimately Fox) is doing something "wrong" here because the orchestration is so close to Coulton's that the artist even speculates that they just took his actual recording and and put the cast's voices over it. And it may well be (I've only listened to the two versions once, but it sure sounds like that's the case). But it brings up so many cases that don't seem so obvious and forces us to ask, "Where do we draw that line?" Or, even, "Why should we draw a line?" It comes back to both the capitalist version of information production (who profits from the production and transmission of this information?) and the romantic author (it's MY orchestration).
This isn't really super insightful or anything. These discussions just make me think of the classical European artists of earlier centuries where this wasn't even an issue. Heck, Handel was well known for reworking his own material, not to mention how others of those artists "plagiarized" (some might say borrowed from or were inspired by) each other's work.