I'm not sure where one would find the roots of the rule but I do see the possibility of there being too much of an advantage to kicking to the sideline without the strong penalty. That cuts the amount of space and improves the pursuit angles for the coverage teams tremendously, which is obviously why kickers usually angle the kick to one side or the other when they kick to cover rather than through the end zone - even with the added risk of the out of bounds rule. It puts a premium on accuracy and gives the return team a half-assed chance rather than just getting squeezed out of bounds. I'm converted.