I can't give the URL here with our comment restrictions. I wrote about Euclid in 2013. a The company claimed does violate privacy because it doesn't tap into calling or browsing data. Also each MAC address is scrambled, "using a one-way hashing algorithm," the company said. In case people are still concerned, Euclid publishes its privacy commitment and offers a way to remove one's data from its site or through instructions posted at the stores that use its service.
Even if Euclid does know your location, "it won't know much more than your cellular provider, or any of the app vendors to whom you have given location permission on your phone." And the fact is that stores already do their best to track customer activity either through loyalty cards, store credit cards, or by asking for phone numbers at checkout.
This kind of data collection is less personal, focusing on general patterns of visits and purchases rather than personal ones. It's not the kind of marketing that Target made notorious with offers for baby products to pregnant women, but an understanding of what brings customers in and how long it takes them to buy.