Faraday cages are fine. It's just never worth the expense.

It's frequency jammers and active blockers which break the law in a massive way.

The reason being you can control the area a Faraday cage encapsulates. A signal jammer that has any decent effect has to also affect outside the area. Big no

Plenty of buildings are accidentally faraday cages for certain frequencies.

Emergency services have training for buildings with poor signals and it's as simple as putting repeaters down as you progress from outside to inside.

Jammers are much less simple to work around.