Because the psychiatric/psychologic industry has become hugely politicized of late, in case you haven't noticed. They want to 'normalize' pedophilia, homosexuality, people wanting sex changes, etcetera. Consider what has been done to boys across this nation who act like boys....they recommend doping for ADD, ODD, ADHD, and a host of other 'kid' behaviors that have all of a sudden become 'disorders' needing 'treatment'.
I predict that the pharmaceutical industry is behind this. After everyone gets diagnosed with something, Compassionate Conservatism will be right there to pay for the meds.
I have yet to read any proposal for mandatory medicating of children.
If the mental health screening looks for learning disorder, mental retardation, autistic disorder, suicidality, and antisocial problems, and it has a system set up for early intervention for these children, Then it sounds like a good system.