Not sure. If someone has a cell phone and needs to call 911, would you be held liable for their inability to make that call?
I've been in plenty of buildings with terrible-to-nonexistent cell reception. I somehow doubt that they'd be held liable if I wasn't able to get reception to make a 911 call. I can't see that the large Faraday cage example is appreciably different.
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.