> Why does HNs readership, a group of people who got their start in computers as kids by likely using technology in ways that were subversive (I.e programming calculators to solve test problems that peers had to solve by hand as intro to programming), seem to support banning cellphones for students?

Because cell phones and the internet can be a never-ending black hole of customized low-effort distraction.

Pretty much everything before had natural limits built in that kept them from being as problematic, e.g.:

Video games: every game gets boring eventually, and it'll be awhile before your parents will drop $45 on a new cart (~$105 in today's dollars); TV: you can only watch what's on, which at least occasionally is nothing that you like; etc.