AFAIK there is no evidence that running is bad for the knees. There is not enough research to draw firm conclusions, but a Stanford study (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556152/) didn't find a higher incidence of osteoarthritis in runners (20%) vs non-runners (32%) after 18 years of running. There is selection bias in this study since the runners were self-selected.
The book 'Which comes first, cardio or weights' from A. Hutchinson offers one hypothesis in which running might protect the knees: by limiting weight gain with advancing age. Less weight means less stress on the joints.
The book 'Which comes first, cardio or weights' from A. Hutchinson offers one hypothesis in which running might protect the knees: by limiting weight gain with advancing age. Less weight means less stress on the joints.