I've always thought about this when walking in and out of a JB. Plenty of times I've got some sort of electronics that I've purchased either from JB themselves or elsewhere in my bag but the bag check people wouldn't suspect anything.
Which leads me to thinking why they'd even check everyone's bags, do they have some kind of special power to determine whether someone has stolen something?
Added HN cause some stores have them too but not like JB where every store has one.
Because its mostly a deterrent
The same way big w/kmart has a greater that does bag checks and costco checks off your receipt
I bet they do catch a small percentage of shoplifters as well