A TV can usually be controlled quite effectively by a parent (if they choose to do so). Access to the internet can not be controlled in the same way. Taking it away completely is often not desirable, fine tuning access is not feasible even for tech-literate parents. Hence a greater responsibility to confirm the age of the consumer should fall on the party making the content in question available.

This is basically the same reasoning why it is legal for parents to buy cigarettes and store them at home, but shop owners can be held liable if they do not check the age of people that they sell cigarettes to.