Coaches who teach their players to hit until the whistle sounds are teaching their players incorrectly and are asking for penalties. A player has a responsibility to know when the runner is down, within reason. A defender hitting a runner who is already OOB. It's no different for a player who is downed inbounds.
As for officials and whistles, the rule book reads, "An official's whistle seldom causes the ball to become dead. It is already dead." The fact is the only time an official's whistle causes the ball to be dead is when an inadvertent whistle occurs.
That's why players are required to recognize when the runner is down.