It merely stops websites like
http://www.d20srd.org/ or doing what
WotC is doing with Dungeon and Dragon - making online magazines (or print magazines, but that's another issue).
It also stops what Monte Cook and others did with Arcana Unearthed and Iron Heroes (True20 also comes to mind) (i.e. stand alone player's handbooks where you didn't need the core rules to play the d20 system). Initially,
WotC thought that merely keeping publishers from using the d20 logo would stop them from reprinting the player's handbook that talks about character creation.
Well, they were wrong and the
GSL makes sure that won't happen again.