But it does. That black line between the approach and the lane makes the body stop. The body does stop, but all that excess force has to go somewhere. That is what makes the body turn. In conclusion, the turned body is a sure sign that the bowler does not have a free armswing, and that a loss of accuracy is the result.
Repetition is the name of the game.
High-skilled bowlers repeat shot after shot after shot with little or no variance. This is not possible until the armswing is as free as can be. The objective should not be to keep the body square at the line, rather it should be to free up the swing more and more until the body fixes itself. Just to let you know, I was able to find a basketball court during my week in Terre Haute, right outside the motel. It was great stress relief to shoot baskets just to wind down.