From 1949 to the late 1960s there were over 100 western series that aired on the networks. Although Gunsmoke was my favorite, every time I watch or listen to an episode on both radio and TV and the narrator refers to him as a U.S. Marshal, I wince. A U.S. Marshal was an administrator. It was the Deputy U.S. Marshal who got to have all the action. A deputy U.S. Marshal only dealt with federal crimes. So, in reality most of the time Matt seems more like a town marshal. I guess the producers didn’t think that title was sexy enough.
All that being said, oftentimes a sheriff or town marshal such as Virgil Earp was also a sworn deputy U.S. Marshal. This greatly expanded his authority.