Here are a couple possible explanations for the oddly popular practice of approaching someone's work station and then staring uninvited at what's on that person's computer screen.
1. The people who do this simply don't realize how obvious it is. They think you can't tell where their eyes are focused.
2. They just don't care if they violate the rules of workplace etiquette. They are so tired of seeing the boring stuff on their own computers that they are willing to risk ostracism in exchange for a chance to read something interesting on someone else's screen.