I have to admit that this does sound like a truthful statement. But if you think about the physics behind force which is mass x acceleration, you can see how this is a myth. The amount of force on the joints (the knees and ankles in particular) is the same whether you are running on a treadmill or the pavement. Your mass does not change from one surface to the other and in so many cases, people actually have the ability to run faster on a treadmill than the pavement. But it is widely suggested by fitness professionals and so many websites that running on a treadmill will lessen stress to the knees.
According to Todd Schlifstein, DO, a clinical instructor at New York University Medical Center's Rusk Institute, "Running is a great workout, but it can impact the knees -- and since it's the force of your body weight on your joints that causes the stress, it's the same whether you're on a treadmill or on asphalt." If you have knee problems or pain it is most likely going to be better managed by cross-training, not by simply changing the surface you log in your miles. However, die-hard runners are hard to change.
I am not a proponent of long distance running as a form of exercise. The repetitive biomechanics do not provide the body with a total body exercise experience and that lack of dynamics keeps me from including it in my personal routine. However, running as one of many different intervals you include in your workout regiment can provide you with balance and perhaps help you avoid chronic knee problems, like patellofemoral (or runner's knee).