Levine speculates that Gillis hasn't been prosecuted because the RIAA is afraid of losing a precedent setting case in which large snippets were found to constitute fair use. I would guess it may also have learned that prosecuting is pretty unpopular with their music buyers.
Although this case seems to constitute an example in which fair use is expanded and copyright restricted, it is only one small example of a possible win against the ever expanding limits imposed under copyright law. Fighting these cases in court against the well heeled doesn't see to be working, so I have come to agree with Larry Lessig that the IP monopolists will only be rolled back by political action.