Route running can be taught. But not every WR can be taught good route running. What Troy has to do is show the receivers what they are doing wrong and work with them to improve their technique. To a certain extent it is on the coach, but the player has to take responsibility for their play.
But we are seeing bad route running across the board and the young players not developing like they should. Douglas isn't a great route runner either. You can just get him the ball quickly and let him improvise. You cannot do that with Thornton though. He isn't that type of receiver.