Tell them to use toes like evolution intended them to, not their heels. The sudden impact through the heel can cause wear and tear for the knees, but the biggest wear and tear is on your stamina really. I watched a documentary on Tarahumara tribe and how they can run for hours which explains this really well.  Oh and if you're a fat fuck, you can break your bones while running - but that goes for about any physical activity. Other than that, I think it's just people being stupid and thinking accidents = running is bad. Like I injured my ankle once running over broken pavement, when I could've just crossed the street. Again, my stupidity caused that, not running. TL;DR running is good for you if you're human, you just gotta do it right.