DBs can't just watch the qb the whole play or there's no way they can follow a receiver. Them not turning their heads is because they are getting beat on the route. Once they figure out the route and can run with the receiver (usually with a hand on them to know they're still with them) they'll look for the ball. If you're beat you're number one goal is to catch back up to receiver and to do that you have to know where they are.
I guess all I'm saying is there's more to it than just turning your head around. Not saying it's not an issue, just that the real issue is occurring before the "look for ball" point