Apps are allowed to track your location in the background. If you go into the Location Privacy settings in Settings, you should see that "Apple Store" is set to

Apps are allowed to track your location in the background. If you go into the Location Privacy settings in Settings, you should see that "Apple Store" is set to always. Additionally, I bet they're using some iBeacons to track when you enter the store. These work over Bluetooth LE and the phone can do different things when it hits a nearby beacon.