Your dog is very intelligent. He knows that anyone who comes to your house is not a threat to him. This is also because you have given him an open environment where he's been able to be socialised with his environment. If you keep a dog confined whether Rottweiler or any other breed, he will feel very unsecured and will feel any thing as a threat to him. Such dogs who're not allowed to explore their surroundings can be unnecessarily protective of themselves or their territory.
I think the below scenario seems to be a little exaggerated. The beggar who came to your house is a daily customer whom your dog socialises every day. A dog would never consider such people as a threat whether it's a Rottweiler or any other dog.
You must have noticed that your dog is very alert about what's happening in the house. He will bark if he notices any suspicious activity. Please remember, he's a pet dog not a loaded gun which you can use against anyone.