That simple principle was the basis for the Federal Trade Commission's (FTC) complaint against Nomi Technologies -- the first enforcement action by the FTC against a retail tracking company.
Nomi's "Listen" service allows retailers to follow people as they move through their store -- or walk past it -- by tracking their mobile devices. Listen collects the MAC address (a unique 12-digit identifier assigned to each cellphone) as those devices search for WiFi networks.
The FTC complaint alleges shoppers didn't know they were being tracked and there was no in store opt-out mechanism for them to use. That could only be done online.
Nomi tracked about nine million mobile devices within the first nine months of 2013, according to the government's complaint.
"It's vital that companies keep their privacy promises to consumers when working with emerging technologies, just as it is in any other context," said Jessica Rich, director of the FTC's Bureau of Consumer Protection, in a statement. "If you tell a consumer that they will have choices about their privacy, you should make sure all of those choices are actually available to them."
In agreeing to settle the charges, Nomi is prohibited from misrepresenting people's options for controlling whether information about them or their devices is collected, used, disclosed or shared.
In a statement emailed to NBC News, the Nomi Corporation (formed when the company merged with Brickstream last October) wrote:
If you don't opt out and you go into or pass by a store with this technology, the retailer will be able to follow your movement and know if you came in or kept walking, how long you were in the store and if you visited another one of their stores.
Keep in mind: Other companies offer similar mobile tracking services. If you don't want to share your location information, turn off the GPS location feature and WiFi when you don't need to use it.