I would not be at all surprised if the feds learned of James "Whitey" Bulger's location well before the latest ad campaign. So why the ads? To disguise the way the feds actually learned of his whereabouts, the timing of the acquisition of said intelligence, and the source of this intelligence. So what they may have done, once they knew where he was hiding, was run a very public and visible campaign involving billboard ads and so forth; they then move in to make the arrest, leaving the world to believe there was an anonymous tip emerging as a result of the campaign.
In other words, the feds are protecting an informant. Who might that informant be? Someone very afraid of being designated as an informant either for reasons of safety or embarrassment (perhaps a friend or family member of one of the two fugitives?).
Nothing is very straightforward when it comes to the FBI. Reagan used to say about the Soviets: "Trust, but verify." I have a different modus operandi when dealing with the FBI: Never believe their press releases. There is always an ulterior motive.