simple 1 company owns napster and napster logs who downloads what and can attach personal information to the downloads this makes it easy for the music companies to go after the 1 company (napster) andd make them shut down there servers / reveal the identities of downloaders Gnutella however with it's lack of logins/handels makes it a royal bitch to track someone unless you use a little questionable software to hack them while they download (not cool as far as any US judge is concerned) and they can't attack the servers since no one company owns them all you hit one company and you may take down 2 servers and with in 5seconds 30 more can be up to replace them.

more simply put both are equaly questionable as to legality napster just got hit because it's easy to hit and a bigger name.