When kids are missing, no one needs to know their "faults" , not once LE has determined they did not run away. With adults, it is vital to know their habits and any illegal or shady activities they may have been involved with and we have seen parents decline to admit these things...it hampers the cases.
Mark seemed eager to point out Dylan's flaws, in his eyes, his carelessness, his being liable to "fall into a lake" or visit the camp with the known sex offender, etc...all things that made his disappearance sound like it was Dylan's own fault. Even his ex-girlfriend joined in, saying Dylan was too lazy to move from the couch. Most unusual, IMO.