No they are not. Labs are used as single purpose detection dogs in K9 units. NOT for patrol work aka bite work.
Single purpose dogs are cheaper and less of a liability which us why some police departments have them. Labs being hunting dogs have a great sense of smell and good prey/ball drive so they make swell detection K9s.
The patrol dogs are always of a breed that was selected to be civil (meaning it has a willingness to bite a person). Shepherds, malinois, and Dutch shepherds being most common. A handful of other herding and guardian breeds can be found but they are pretty rare.