the rules for the 100% shouldn’t be made because 10% can’t self regulate.
Unfortunately that's hard to avoid, because those 10% will disturb lessons and take up the teacher's attention, thereby negatively affecting the other students.
Are the teachers supposed to do extra work to ensure no teen had a cell phone?
It's way easier for a teacher to take away a phone that disturbs a lesson when there are not supposed to be phones in the first place, than have to argue about exceptions and limits to the rules every time.
I agree and sympathise with your overall philosophy, but I'm also conscious of the practical limits, unfortunately.
Phones now are a way more important part of people's (and especially teenagers') lives than they were back then. And they're often also used to support lessons.