Do you really think there are expenses?
A single entity can gain the contract for wireless in all of a particular operator's malls. Say, the Mills malls. That's say, 4 malls in the Maryland region for which one operator could potentially connect. The wireless operator scores a contract to install wifi. They can work out a deal where the wireless operator can work with the mall to provide coupons for various stores inside of the mall and work as a central mall hub. They can make it appear like it's helping the shopper out! "Sign up for a Mills Account to earn great deals during your shopping experience today!"
Next thing you know, this vendor is keeping tabs on your authentication to MAC address storage. Even when you visit other malls where you might not have an account in that mall, they can still track your whereabouts because hey--they have your MAC on file. Got a different phone? No big deal. As soon as you sign back in to your "Mills Account" from the new phone, the tracking starts anew with a new device.
Then start using some of those coupons...
Before you know it, they've collected a massive database of your shopping habits. AT the very least, location tracking. At worst, intimate knowledge of which stores you like to purchase from.
Let's not forget what can then be done by analysts with access to that data. Like to shop at Spencer's and Victoria's Secret? I bet you're a freaky girl in touch with her sexuality, not conservative.
Think the UNICRU test that Best Buy employed for personality-based hiring to the extreme. A complete profile that you've built for yourself through all of the websites you have visited with "SHARE THIS!" links (Even if you don't actually share it, they're tracking.)
What the hell do you think "big data" is?