Are they posting music and demos on these blogs? (Not any more...)
I assumed that what was being objected to was writing about music before it is officially released ("under embargo"), or, well, at all.
Or, writing about artists' drug abuse, violent incidents, and lousy performances and releases. That isn't necessarily breach of copyright or confidentiality really but it isn't welcomed by music publishers either.
Then again, I suppose sampling in music could be a legal problem, and in demos or DJ mixes particularly. Creative people may clip and use identifiable material that they don't have the right to use.
Then again, let's randomly select for instance Eminem. Suppose that he releases a track describing how he intends to commit a major bank robbery, in detail. Then he does it. Due to the DMCA you wouldn't be allowed to say what he'd done - well, not online.
I just hope the police are paying close attention to the pop charts.