If the cop was in the bed of the truck and shot into the cab, that's a pretty easy shot to take compared to trying to hit a moving vehicle. The bullet also is also either going to end up lodged in the person or the truck, as grim as that may sound. If they tried to shoot the tires from the side and hit the pavement, the bullet has a greater chance of ricocheting and won't stop until it hits something.
I don't agree with the officer's decision to shoot in the first place, I'm just saying that going for the tires is also reckless and dangerous. They could have tried to pursue the fleeing truck, though that has it's own problems and risks associated with entering a high speed chase with a potentially drunk driver.
