Hey, something I'm somewhat qualified to answer! So, yes, these systems are actually effective. The systems and procedures are designed to look low key, but essentially perform PRISM-like mass surveillance behind the scene. These systems are managed by former US IC personnel.
What happens is that your identity is tied to these purchases and after a certain threshold you get flagged as a thief, essentially. At that point, you will get very increased attention (via checkout, purchases, and floor walkers), and after another threshold, will be trespassed and/or prosecuted.
But, you'll probably get away with a banana or few before you trigger the loss prevention threshold.
What happens is that your identity is tied to these purchases and after a certain threshold you get flagged as a thief, essentially. At that point, you will get very increased attention (via checkout, purchases, and floor walkers), and after another threshold, will be trespassed and/or prosecuted.
But, you'll probably get away with a banana or few before you trigger the loss prevention threshold.