They are

http://lifehacker.com/how-retail-stores-track-you-using-your...

They use it to track your movements in the store. I forget the name of the most popular provider.

It's called Wi-fi Indoor Positioning. Last I heard, the state of the art uses triangulation of signal strengths between multiple access points. This gives it 'aisle-level accuracy', or about 5-10m.

It's pseudo-anonymous in that they can get a unique identifier for your device (and thus know how often the device returns to the store) but can't tie it to your real identity without more information.

And given the mediocre accuracy of the technology it would be hard to correlate it with, for example, their point of sale system (e.g. 'device XYZ was near checkout 3 at the same time that John Smith's loyalty card was used there, therefore device XYZ is owned by John Smith').

Assuming John Smith comes back to the store a few times and you're willing to crunch the data, I think it would be very doable.

So what if came to store with counter signals, broadcast fake MAC addresses, SSIDs ?

Less technical know-how needed to get customers to sign up for the store rewards card, and no one will complain about you using that data.

I've been asked for an address when making a $5 cash purchase. I have the baseless impression that it is marketing seminar advice that small specialty businesses get (That is, collecting addresses).