If this can indeed be a mandate instead of just "demand parental controls", the issue would be inflexibility. For a lot of people, limit would be enough. But for a lot, who are using their phones for actually beneficial purposes, it is not, and such people (who can also be talented and useful) are completely disregarded by universal rules like this. I myself firmly believe that smartphones are not suitable for actual work, but every situation is individual, for a lot it might be genuinely necessary. Also maybe things like LineageOS could bypass this anyway.