The big difference here is that purchasing a porno magazine requires interacting with a clerk who will then verify your age and send you on your way. A better comparison would be banning the sale of all porno magazines unless the buyer registers for a porno card for his/her family (which would then provide unlimited access regardless of age).

I tend to agree that unfiltered access to all sorts of things can have an unhealthy impact on the minds of kids, but this is still the wrong way to handle it.