Well, I say sack the police officer if I haven't yet.
The shoe can't possibly have hurt him, there's no real energy or force behind it from what can be seen in the video.

The trainee, well, either he is a scumbag as well or he just knew that if he didn't do what the other guy did he might lose his job/be considered a rat etc. That's group behaviour/esprit de corps/whatever it's called for you.