This is because when asked to explain why it is that children who they falsely describe as having always gotten along with suddenly become defiant, John and Elly blame The Media for filling innocent heads with scary information that should have been censored. This makes sense to them because their brains are like sieves in a way. Any memory that might lead them to the conclusion that they've never been able to cope with their kids sluices down a hole and thus leads them to the conclusion that suddenly, the children that they've always related well to become surly jerks. Since they've noticed that the kids watch a lot of television, television must be to blame and NOT THEM.
This sort of nonsense is akin to Elly's refusal to remember that she's always hated what she sees in the mirror in that the two of them are trying to avoid facing up to the fact that they've always acted as if they've been fighting for their lives and always been wrong. Rather than admit "Hey! We're two very thin-skinned people who don't like being told 'no' even when we're in the wrong!", they need a fall-guy with whom they can play the game "Beg the question." Simply put, scary information is scary because information is scary. From their perspective, that makes sense because the scariest of all information is that they themselves are mostly (if not solely) to blame if their children do things that trouble them. Since they don't want to admit that they've set their children a horrible example, they have to blame the Media.