This is a very good question. They may have needed the helo out of the way to pull off the whole event. For example, if it was orchestrated then they most likely would have had to switch the driver out after the incident. If a helo was in pursuit that may have been difficult to pull off. The following link has some 4Chan images of the car after the stop where it appears that a witness states that black SUV's were outside her home telling them to get back inside when the driver got out of the crashed car and a different person got in. These images raise additional questions. The images have been saved but the 4chan post is no longer available. The helo police would likely have seen this occur, had they lived. https://steemit.com/informationwar/@stevescoins/4chan-already-uncovering-the-charlottesville-car-attack