Most thefts by customers are more opportunistic, the visible cameras have much more effect on that. Staff theft tends to be more pre-planned when it happens, so they'll work around the cameras if they easily know what they can see. Same for break-ins to employee-only areas (warehouse etc.).

The threat model in the two areas is a bit different, so different tactics are used. It doesn't mean that some people are trusted more than others, just that as groups they behave differently.