Because they are capable of accomplishing the tasks that are demanded of the role to a level at least adequate as determined by the organization, and have the personality traits necessary to give at least adequate trust that they will carry them out under pressure. Simple as that.
Keep in mind also that not every working dog you see is being run by special operations forces handlers with decades of institutional experience. There is wildly different levels of ability, professionalism, competence, and experience across this country. From podunk PD that let a deputy who really liked dogs and did internet research acquire a puppy and train it himself as the counties K9 unit (that's a true story going on right now, the dog is little more than probable cause on a leash) to SEAL Team 6 helicoptering in to Pakistan and taking down Bin Laden with Cairo the Malinois.
That being said I suspect your real question being: why if slope backs are a bad thing are dogs with what you see as sloped backs doing working jobs?
Because to a certain point it doesn't negatively affect the dog and the trait is so ingrained into the breed at this point that you'd be very hard pressed to find a nearly square GSD, any GSD. (Yes I know GSDs are longer so they'd be more accurately described as a rectangle than a square but I'm talking as close to a square as a rectangle can get). Much less find a perfect stud and dam pair in all other aspects that throws great pups, that are also very squared off, to make these pups in the first place.
Steps should be taken to minimize this trait as much as possible without sacrificing what makes a GSD a GSD, but in the grand scheme of things it's here to stay OP. Put your money where your mouth is, just like alot of us did myself included, and financially reward breeders who breed dogs of solid temperament with good structure. Encourage others to do the same as well, but also recognize that some slope is inherent to the breed at this point.
