They can't even prevent themselves from being swiped. We recently heard from a lady in the Midwest U.S. whose homeowner's insurance policy was being cancelled by her carrier because her dog had nipped a neighborhood child. Businesses that put guard dogs on duty on their property also are taking a huge risk from a liability standpoint. Any professional burglars will quickly dispatch dogs if they really want in. Only juvenile delinquints might attempt to run the Doberman gauntlet, and if they happen to be mauled or killed it could be financial lights-out for the business establishment.
Legal liability is one of the heaviest arguments against protection dogs.
Unfortunately, the justice system is stacked against the dog that bites. Moreover, protection dogs are not nearly as much fun as affable, social dogs. They can't be taken everywhere. They have to be locked away when company is over. They usually don't become as companionable as do well-behaved dogs.