Actually people don't give a shit who work there (a big chunk of the staff steal produce or write it off and take it home as well), there are plenty of other shops to choose from, the police don't care (in the UK they send you a letter if you get mugged apologising for not coming out) and the business find it costly to deal with the people. It's only significant if it's more than a few pounds/dollars which is why high value retail tend to have competent security.
When capitalism pushes profit margins, some losses are insignificant not to invest in otherwise they will hurt the margins they are trying to protect.
Also if they gave a shit, they'd have staff on all checkouts and not use self-service checkouts. Staff are more expensive.
They know; they just don't care.