With all the surveillance in those places that also helps with the logistics Amazon would probably be able to track what was being stolen before it even made it out the door.  Also thse places don't have big glass windows.  Amazon wouldn't have to worry about the optics of having armed guards at the facility because they are worried what kind of impression it would make on shoppers.  Amazon is one of the companies that low key supports this kind of looting that destroys brick and mortar retail - don't think they haven't planned for this.