Great question and the answer lies in the speed differential. Running at the gym on a treadmill makes it almost impossible to focus on reading. Walking on a treadmill at 1.2 mph is actually about a third of normal walking speed and makes reading magazines, books, computers, Kindle's etc. extremely easy.
As others said, it's all speed-- 2.3 mph is about the maximum at which I can still operate a mouse on my computer, for example. The intention, obviously, is not to be able to use a computer while sprinting 8mph, but to convert time normally spent sitting into time spent walking gently. That being said, walking 2.3mph for 90 minutes while playing, say, computer games, is 1) a great way to lose yourself completely, and 2) a sweat-inducing workout.