I believe that the majority of people who read textbooks at Starbucks are actually studying. People learn well in different environments. Some people need background noise to be able to effectively study. I do not really think sitting in a Starbucks with a textbook makes much of a fashion statement and most people go to a bookstore to read or shop.
Putting a textbook in front of your face is hard. Even if you are just trying to look cool, at the very least you're still getting some studying done. There are many reasons why people study at Starbucks, and it's not just to be fashionable. There's good drinks, air conditioning, and you can go with friends.
People at Starbucks are making a fashion and societal statements by going their with their algebra book to study. I feel they are just giving themselves a possible reason not to study. They are hoping to meet someone to talk to and avoid studying without feeling guilty. It just a hipster thing to do.