The thing is that they can track this over time ... so even if they registered all the dots in the area at the time, if they do this often enough a pattern will

The thing is that they can track this over time ... so even if they registered all the dots in the area at the time, if they do this often enough a pattern will emerge where you are the only one with that specific radio ID that makes a certain purchase. So unless you can get your friends to come with you every single time they only need to get data from 3 visits or so...