The theft is not as clear cut as someone pulling money out of the till. Often it involves say, double collecting money from two patrons and only entering in one order or charging a patron for a premium spirit, filling out an order for a cheaper one and then pocketing the difference. Then there is the ever constant overpouring or simply giving away free drinks. With nightclubs there can be all sorts of theft and graft at the door when collecting covers.
Even in bars that don't have cameras, don't believe for a second you aren't being watched just to ensure you're getting your drink quickly or you're not being too aggressive or any number of things that happen when liquor and people mix.
Internal theft. Shrink.
Do what minimarts do and mount a camera pointed down at the till, but don't bother with entrances and exits. It's not a casino.