You may not be able to tell what your children is doing 100% of the time but it seems to me that its a lot easier to control the distribution of money at the parents end. If your kids don't have money then they can't buy the game or have to ask you for it.
The exception IMHO is if kids get a job. Ultimately, if they get a job and make their own money then they are already showing the signs of maturity to becoming independent and have the right to make a decision with the money they make.
I would never tell anyone how to raise their kids and every child is different but there are sensible alternatives out there then making more laws.