By making sure your steel-reinforced concrete mall walls are thick enough to block proper 3G/4G reception (no, really, reception is pretty bad in a lot of indoor places).
But still, I agree, it would be very hard to have everyone connect to your wifi.
it's hard to get everybody, but it's easy to get a significant sample. just set up an open network called Starbucks WiFi and watch all the iPhones connect to it
But still, I agree, it would be very hard to have everyone connect to your wifi.