Why He Needs a Nickname:
Because no one knew his actual name.
For the first few days of this scandal, no one knew much about this guy, except that he'd sent a shirtless photo of himself to another minor player in the scandal. No one knew his name, and seeing as "shirtless FBI agent" was both accurate and memorable, well, there you go. Buy a shirt if you don't like it, buddy.
This happens on occasion when a figure becomes known, usually as a result of his or her actions, while still staying relatively anonymous. Needing to describe this figure a little more succinctly than "the guy who did the thing," a writer will come up with a shorthand label, typically related to the figure's most noteworthy action or attribute.
A great example of this is Deep Throat, the famous informant who tipped off reporters to the Watergate scandal. Yes, I know that the original reporters knew Mr. Throat's actual name and were just protecting his anonymity. But any other reporters who wanted to talk about the guy were stuck, left with no choice but to fire up their noisy old school word processors, crane their necks way back and tap out "Deep Throat."
"Why is this damned thing always so sticky? WOODWARD! BERNSTEIN!"