Why did police stop dally on route to the hospital?

Police stopped Dally on his way to the hospital because he was involved in a situation related to a violent incident. Dally had been injured and was in a distressed state, but the police needed to assess the situation and ensure public safety. Their intervention was likely due to the ongoing conflict between the greasers and the Socs, which Dally was a part of at that moment. Ultimately, their actions were aimed at controlling the immediate circumstances surrounding the incident.