I'd like to know more: Did you hear this from your son, or another parent, or directly from a teacher or the school (written in a newsletter, etc.)? If it's your son or even another parent, there may be more to the story that you're not hearing, like there was a week or two of temporary no-run recess after some kids got too wild or something like that. But if you haven't yet, I'd find out from the principal directly.
If it's accurate that all recesses are no-run zones, I'd want to know precisely why. Is it because, as another person who posted mentioned, the entire play area is "blacktop" or paved, and they fear kids falling on the hard surface? Was there some specific incident or injury that made them stop all running for everyone? Was there a lawsuit from a family whose kid got hurt? Is this a policy just for your one school, or your child's one class, or for all classes at the school, or all schools in the district? You may already know all this and just didn't mention it in the post. But those are all questions I'd have.
Once you have the answers, you can protest it even better. I wonder if it's truly no running at all, ever, or if instead it's really a ban on running games like tag. I have not heard of a no-running-EVER policy, but I have heard of schools banning certain games like tag if those games consistently became rough or ended up as kids just crashing into each other instead of actually playing the game. I'm definitely not defending a no-run policy! But find out exactly why, where and when it all started, then you can dispute it point by point. I think teachers love kids to have a very active recess because it burns off the wiggles.