I think it's that people who've never faced a security threat and never had to think to are among the most vulnerable. And grocery stores are notoriously thin margin businesses who actually strive not to spend unnecessary the most.
I have a facebook extension that alerts when a friend vanishes from my friend list. It does a lot of other stuff too, like remove ads etc. But that's one of its features and it's super easy to detect apparently. Facebook doesn't notify you for obvious reasons, but if you simply maintain a list of friends between visits it's easy.