The OP talked about BYOB, which rarely includes "root access" (either via a root cert for decrypting traffic or admin level access to the machine)..
For BYOD, I don't know what you're gonna do. Many students have smartphones too (some with tethering), and you can't control what they look at on those either. Plus, even if the school could somehow magically lock everything down 100% within the confines of the school building, the students can still get access to whatever at home, or using coffeeshop WiFi, or whatever.
That's fine, these are not school responsibility. Once the parents complain, you can redirect them to their home or coffeeshop.
The point is stopping them downloading this stuff using schools property or infrastructure. If they download it elsewhere, it is someone others problem then. If someone complains, its someone elses' fault, and the school can fingerpoint.
If they just bring it to the school, they can be disciplined, but no other steps need to be taken.
Preventing cell phone calls could have dire consequences in an emergency, and stopping kids from looking at porn doesn't remotely merit taking that risk.
To your point, there are certainly countries and jurisdictions that do not permit blocking RF or have strict exceptions.
In the US, local statutes and AUPs don't enter into it. This is federal law. You're right, there have been a number of organizations that have done this -- and enough of them have been fined for it that the number is much smaller than it used to be.