Why would you need facial recognition to print a coupon? If the system knows that product A is next to product B, then it can just print B's coupon whenever A is scanned at checkout.
You don't. The system needs to be able to track an individual throughout the store, and can do so via several measures. If you're just tracking a person as a persistent-yet-moving object, there isn't much of a privacy concern. However, if one elects to pile on technologies like facial or gait recognition, then one can track consumer behavior across multiple visits. It's another datapoint which carries it's own value from a marketing perspective.
It just-so-happens to also have value to those charged with loss prevention. If a shoplifter is caught, they can be informed that they are no longer welcome to shop in this store. Enforcing that can be difficult unless....
