Unlockable difficulty levels

I am a huge fan of games with adjustable difficulty levels. I think that every player is different, and so what is just the right amount of challenging for one guy might be trivial for the next, or frustratingly hard for a third. Setting ones own difficulty level maximizes fun for everybody. Now what I don't understand is the trend towards forcing players to work for that feature. I'm currently playing Borderlands 2, which starts out as normal, and I'd have to finish normal to get to True Vault

World of Warkraft...