"minimum of knee height" might have been a better way to go about it. Keeps it friendly for those less flexible players and realistically, how much time does it actually take to re drop it a few times if it keeps rolling away.
Besides, the vast majority of players would opt for the lowest option anyway so the same result would be achieved regardless.
The intent of the modernized Rules was to eliminate the "drop, drop, place" procedure. Placing a ball rather than dropping it is always an advantage. If the Rule said that knee height was a minimum, players would be dropping from shoulder height to ensure that the ball rolled outside the permitted drop area and they would eventually get to place it.