To keep people who don’t have a means to pay at that store by having the app installed and configured with their account away from the sales floor.

It’s one thing to stop a shoplifter at a normal store. Turnstiles reinforce the notion that this facility actively enforces access control.

This is similar to Passport Control line at the airport. If I have Mobile passport (MPC) app installed, I use a dedicated and usually empty lane. The same is true if I have my Global Entry card. It takes a couple of minutes to setup MPC while walking, but most people choose to wait in the main line.