This sounds like a racket for residential properties. Alarms do nothing to prevent burglary. Where this is a requirement, I'm sure the insurance company gets kick backs from companies that make or install them. Or it's an easy out, designed to make it as hard as possible for people to get any value from their insurance...
Alarms usually don't prevent burglaries, but they often reduce the amount of theft, as the burglars take what they can do in one trip and leave, rather than comprehensively emptying the building/unit.
