I think the central complaint would be that the site in addition to taking traffic away from Youtube, was then serving ads to users of the site thereby obtainin

I think the central complaint would be that the site in addition to taking traffic away from Youtube, was then serving ads to users of the site thereby obtaining revenue. Google probably have no issue with individuals using tools to download mp3s (or at least realise there is nothing to be gained in pursuing action) but when third party sites are intentionally diverting revenue away, this is bound to cause issues. The legal analysis is interesting in terms of copyright infringement. Particularly as to whether the existing method used by Youtube is a 'technical measure' designed to prevent infringement. I think it arguably is as a large majority of users will not download videos direct as a result of the system used by Youtube.