Banning phones is simpler, and has no downsides. You are at school to learn, you don't need a phone or tablet for that.
Besides, lots of things can be addictive and distracting. Personally I'm addicted to reading news. Had smartphones existed when I was a kid, simply banning social media and games wouldn't have been enough.
Well, people do need to reach you and people mostly aren't chained to their desks in an office any longer so you do need a phone of some sort. No, you don't need a tablet in general and I mostly never used one in a professional capacity.
Besides, lots of things can be addictive and distracting. Personally I'm addicted to reading news. Had smartphones existed when I was a kid, simply banning social media and games wouldn't have been enough.