What is this rule designed to do?

For the life of me I can't and don't understand this rule. I can understand if a player is grabbed by the helmet and pulled down but why the shoulder pads? The rule looks like anytime a player with the ball is in front of a tackler and is tackled above the waist a Horse Collar tackle is called. Just my opinion but I think it is a BS rule. Why is it ok to tackle a QB by the shoulder pads if he is in the Tackle Box, but a penalty when he is outside the Tackle Box? Todays game is almost like 2 hand Touch football.