And perhaps the fact that people will be scared to write down what they think because it might be disapproved keeps them from including the dislike button, because the writing down what you think and/or are doing part is basically the heart of the site.
Look at Gametrailers. Write a comment -- anything! -- and there'll be people giving you thumbs down for no reason. Now if that happens on a social site like Facebook, people will feel horrible when they know their friends can see someone disliked something they wrote down.
It's perfectly logical why a dislike button would not be included, from that perspective.
Playing: Skyrim, World of Warcraft.
Future: Dead Space 3.