My understanding is that these are already banned in most schools and the practical difference between enforcing this at a state or national basis is basically nonexistent vs simple local enforcement.
"Banned" is only meaningful if there are consequences for defying the ban. My experience as a high school English teacher in a handful of schools across several states was that admin is, generally, unwilling to implement a hard ban on smartphones because a significant portion of parents would vocally object (to put it mildly).
Pushing the ban to the state level acknowledges the broad inability of district level leadership to self-police these problems.
If anything I suspect it helps reduce the adversarial relationship between local schools and their parents. "State law, not our policy, sorry." Sounds childish, but hey, if parenting has taught me anything it is that plenty of people never outgrow childish behavior.
See also https://en.wikipedia.org/wiki/Subsidiarity
Any ban above school level is silly.