From a PR perspective having armed security at a grocery store would invite a whirlwind of bad press and unfair comparisons. Most people would agree that stealing food for sustenance can sometimes be justified.

I can already imagine social media backlash over someone being apprehended at gunpoint after trying to steal a small amount of food. Contrast that with how it would look for someone apprehended at gunpoint at a bank. Almost no one thinks it’s okay to steal just a little bit of money from a bank.