Why in the world would someone need a protection dog when they have a 125 pound Rot?

I bought a German shepherd that was suppose to be a protection dog and they brought him out stayed 45 minutes showed me the heal sit and down command then 10 days later after keeping the two males separated one created one out he came back and said when the protection dog is out keep them both on a leash and if they try two smell each other hit them on the nose with the end of the leash well after 3 fights and one bad where my 125lb rottie was determined to be king everything was fine and after 2 weeks the dog just up and tuned on me on night for no reason and did serious damage to my face and arm and had it not been for my rottie I would not be sending you this email how long should i have trained with this dog before letting them bring him into my home