You might have been mature enough at age 16 to watch any movie, but what about those who are not? And where is the line? 15? 14? What about 11 years old? The point is not about mature, the point is there must be some line. And even with a law denying 16 year olds access to R-rated material, they still could get access if a parent made the purchase. It is not set in stone.
I don't think it is asking too much to have laws which let the parents in on what the kids are doing. Everyone on slashdot knows that most 14 and 15 year old hide what they believe their parents will not approve of. If the 14 year old knows dad does not want violent games, that game will be hidden and played while dad is at work. By having a law which will not allow sales to minors without a parent, it gives dad the upper hand.