The point of the faraday cage is to explicitly allow specific signals by setting up repeaters (i.e. femtocells, or whatever they're called these days) inside the cage.
Do they allow of block data connections over their repeaters?
If they block them then customers are pissed. "Why doesn't my email work? Why can't I get picture messages?"
If they allow them, people can set up wifi hotspots the same as they were before.
Revenue lost from pissed off customers + cost of Faraday cage + cost buying and maintaining repeater network is definitely going to be higher than revenue from selling wifi.
