Surely that would affect mobile phone reception. I can't imagine anyone being happy about that.
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.