They're banning smartphones, not (say) laptops. Presumably the latter are less of an issue since you're not tempted to be using them constantly at all times.
For many people the only computer they have is their phone, I’d say well over half the people I work with have only their phone, and especially young people.
I’ve got one, two, thr… I don’t, maybe nine computers, and have read zero on any but the phones.