Loyalty cards are opt in. Security surveillance can be unsettling but customers understand its purpose and limited scope. What you're proposing is more invasive, and most people would not appreciate it if they knew about it.
Look, give up trying to justify it. Customers don't want it. You should find another application for this technology.
Well, I definitely do have other applications for it. For example, I know that similar software has been used in labs to estimate people's reaction to videos and game features, in mobile applications to improve interaction with a user, etc.
My interest to offline applications comes from personal experience: recently we demonstrated our product (not emotion recognition, but also capturing user's face) on an exposition. People came to our stand, used the product (so they clearly opted-in), asked questions, etc. After 2 days, we asked a girl at the stand "What do people think about the product"? "Well, in general, they are interested" she answered. Not much info, right? Definitely less informative than "65% expressed mild interest, 20% had no reaction and 5% found it disgusting, especially this feature".
So I don't try to justify this use case - my life doesn't depend on it - but I find it stupid not to try to understand your clients better when it doesn't introduce a moral conflict.
Look, give up trying to justify it. Customers don't want it. You should find another application for this technology.