As jschillig tells us, the guiding principle behind this is the unswerving belief that whatever a child might want must always be the very worst for him. Never in the strip's history is there the least hint that Alleged Parents John and Elly trust their children to make the right choices in life because the shibboleth that guides them is the belief that children always and ever make the worst possible choice. We are thus implored to sympathize with two poor, misunderstood parents who don't mean to be so judgmental but are forced to by ungrateful kids who can never be expected to do the right thing, wear the right clothes, want the right career, listen to the right music, eat the right food and marry the right person.
The reason that this is not only irritating and exceedingly arrogant of them is that History teaches us that what Brokaw calls the Greatest Generation celebrated their victory over Hitler by vomiting forth the stupidest generation of craven, greedy, soft, weak and entitled goose-steppers to have ever blighted an unhappy world. John has the sparkling intellect of roadkill, Elly's brain is simply two neurons held together with duct tape and between them, they've made any number of catastrophically stupid mistakes based on an ignorance so profound, it's dialing down the collective IQ of humankind to the single digits but yet those two morons think that they know better than their children how they should live their lives.