Currently, basic cameras and alarm systems are very affordable, more so than most of the staple devices in any given shop, e.g. cash registers. And if we are talking about actual deterrents, then they should be looking at steel window grills.

But let's assume that it is true that keeping a store lit is a deterrent, then why wouldn't they put up cheap, motion activated lamps, around the store?

Point is, businesses need to keep up with the times. Just stating that lightning keeps thieves away is not going to cut it.