Outright bans are because government bodies are scared of nuance. You can also see this in “zero-tolerance” policies that do things like punish the victim because they were “involved” in a fight, or punish a kid who nibbles a chicken nugget into the shape of a gun.
To be fair to schools, nuance is hard. Suppose that the rule is “phones may not interrupt class.” Now, what counts as an interruption may vary between classes, between teachers, and based on what’s happening in class. A student may use it during a quiet period in the class when they’ve already completed their work, and that’s acceptable. A different student will then use their phone ten minutes later, when they’re supposed to be doing something. The second student will get in trouble, but then complain that the first student didn’t get in trouble. The parent will hear, “Brayden was using his phone and he didn’t get in trouble but the second I used mine, I got in trouble. The teacher has it out for me.”
If you’ve talked to any teachers in the past few decades, a common theme is parents siding with their kids against all logic, reason, and evidence. They’ll assume that teachers are petty goblins, just looking for an excuse to pick on their kid. And parents can be outright hostile and unreasonable. When my wife was a teacher, she received more than one actual death threat from parents because she enforced rules that did NOT have any nuance or discretion. Imagine if enforcing the rule was up to the teacher’s discretion versus an outright ban.
tl;dr I agree that a ban is silly, but I totally get why schools are doing it.