Two of whom were (known to the producers) exaggerating their own experience and knowingly using Kellee's experience to further their own games, one who explicitly told them not to intervene, and one who was only going by the other's word and didn't really agree that there was bad intent. Unfortunate, but true.
You could go on forever, but neither physical assault nor car theft nor murder etc. are brushing a hair out of someone's face / putting an arm around someone. The point is, even with an actual crime committed, a complaint typically needs to be made for an arrest to happen. Police aren't trying to send a case in front of a judge with no cooperating complainant which will just waste time and money.
There is no doubt something about ejection if you're harassing other contestants, but likely not enforced without corroborating footage (which they had here) and a formal complaint (which they didn't have here.)