I'm guessing in court they have to say their full name, and the media uses that to protect anyone with the same first and last names.
Or more realistically, to protect their business & profits from
lawsuits alleging
defamation. Generally speaking, there's no legal requirement that a lawsuit have legal merit for it to be filed by a plaintiff, forcing a legal response from its defendant(s). The full name might be mandated by editions of the
Associated Press Style & Libel Manual.
When not relevant to a crime, I've long considered the routine use of a 3-full-part name to be an
affectation of the
ruling classes of New England & New York, e.g.:
Charles Nelson
Reilly (actor/comedian)
Franklin Delano
Roosevelt (U.S. president, albeit before I was born,
and reportedly often simply "F.D.R.")
Henry Steele
Commager (historian)
John Foster
Dulles (lawyer-diplomat, later U.S. Secretary of State: 1953--1959; namesake of Washington Dulles International Airport)
Richard Henry
Dana (seafaring author)
John Jacob
Astor (Palatinate-native German, immigrant fur-trade magnate, "first multi-millionaire" in U.S.A. per Wikipedia)
John Quincy
Adams (U.S. President; his father was merely U.S. President John Adams
Jr., Quincy being the family name of
Jr.'s mother-in-law)
Samuel E
liot
Morison (historian: What other "Samuel Mo
rison" did he fear being confused with?)
But it's not exclusively a Yankee affectation, e.g.:
Lyndon Baines
Johnson (Texan, U.S. President following assassination of J.F.K.)
John Noble
Wilford (Kentuckian, but master's degree from Syracuse U. (N.Y.), then Pulitzer-Prize-winning science-writer for
N.Y. Times)
Hans Christian
Andersen (Danish author of fairy-tales)