Well, like the encounter says, it's mostly just to set the tone of how bad the violence has gotten. Whether the PCs engage the punks is almost secondary. They can if they want - and/or if it makes sense for their character - or they can just take cover and stay safe. Both are entirely valid options. I'd only stop the car if the PCs manage to stop it. (Shooting the tires, shooting the driver, planting an Earth elemental in the middle of the street for the car to crash into, etc.)
The point is to set up the premise that the violence in the city is so bad that the PCs are getting caught up in it through no fault of their own. Sure, they're likely violent criminals themselves, but the drive-by is random violence, not the controlled, precise, professional violence of the PCs. So if the PCs don't bite, or tend to the wounded, or some other non-violent course of action, let it play out rather than forcing the fight. This is a chance to let the characters emerge.