The FCC begs to differ:
https://www.fcc.gov/document/warning-wi-fi-blocking-prohibit...
A purely passive barrier to electronic signals is a completely different thing. There are no rules against Faraday cages, and the FCC is perfectly aware of them and what they are for; it OKs the use of Faraday cages for use with experimental radio technology that would otherwise require a special permit.