I learned that at combat training; up until then I assumed they were basically rubber paint balls with a higher velocity. We asked why we had to use chalk rounds for training (basically a 9mm pistol powder charge with a chalk bullet in front of it) instead of rubber bullets, and our combat instructor laughed and said he didn't want to see anyone blinded, unconscious, or permanently damaged, so they were a no go, even when we were wearing ballistic vests, helmets and face shields. Those chalk rounds still hurt like crazy, and would break skin through military camo uniforms; can't imagine being unloaded on with rubber bullets without protection.

So, basically these cops don't actually know how to use the weapons they are wielding? If they used those rubber bullets against realistic targets they would learn how to use them properly.