When I was in 4th grade, I was running; I guess playing tag and my best friend pushed me hard - my head hit the monkey bars (but back then they were metal); it knocked me out, I fell to the ground but quickly came too.
I felt this crumbling; it was one of my front teeth - it was broken in half, and I have had untold dental bills since then trying to keep the tooth looking nice. Luckily the nerve was not broken or I would have lost the entire tooth.
I have a veneer now and you can't tell, but my mother was totally hacked off that that happened.
Unfortunately some of the kids can get just TOO rough and push too hard. It's mean and sometimes passive-aggressive. I don't know why kickball and other games wouldn't be allowed though.
It's very sad but true that some kids have absolutely no empathy for what might hurt another child, either physically or mentally - there are bullies that act like you're playing tackle football; and they may even laugh when someone is pushed too hard and gets hurt. Or they'll get defensive, "I didn't push her that hard; she just fell?"
Maybe really good playground monitors could keep things under control, but here they expect the teachers to do it; and teachers need to sit down and rest a little, and can't be in control of everything on the playground.
Also there is the liability issue to consider, for the teachers now and the school. Sad, but true.