Are you allowed to use an umbrella to hit another human being? Of course not.
There are so many levels of hypocrisy here that it isn't even funny. Reverse these roles, and the "ambush" journalist sent by Bill O'Reilly would have been flailing on the ground and would require hospitalization.
The simplest answer is usually, but not always, the right one. O'Reilly objected to being filmed and he used his umbrella to assault someone. Note that the videographer was NOT arrested at the scene and that O'Reilly's umbrella was ruined.