"Isn't porn illegal for you if you're under 18? What? Not if your drunk ass redneck dad says so? SWEET!"
Actually it's not illegal if your parent or guardian approves it. It is only illegal for an unrelated third party to sell pornography, cigarettes, or alcohol to a minor. It is also illegal to sell R rated movies and M rated games to minors. Ever have a glass of champagne at a party with your parents, let's say great aunt Edna's 97th birthday or something, when you were underage? Try that on your own and there's trouble, but if your parents give it to you there's no problem at all. Same idea.
It is not illegal for a parent to buy one of those things and give it to their child. A poor decision? Probably. Unpopular? Obviously. Illegal? No. And in cases where a parent does cross the line, that's where child protective services will take the kid and throw the parent in jail.