The principle of "No Logo" is to not give corporations free advertising, unless you've decided they deserve it. Most corporate logos are tacky and uninspiring, yet people wear them anyway. Large ones. How many people who have Nike or Adidas gear think about that company in the same terms they do, say, their favorite rock band? Yet, they are supporting that company in the same manner.
The more you look, the more logos you notice. It becomes like a game. How many logos can you destroy?
Note: this is also the name of a book on consumerism (No Logo by Naomi Klein), and a website, http://www.nologo.org.