because who would sue Mr. King?
Far more people than would sue you or me. He has more money (certainly than me, your mileage may differ), so is more worth suing.
Observing writers like Forsyth and Le Carre, I see that in a story set before the next election, if a major political figure only does something completely predictable (denounces a terror attack, presents an award) they will often be mentioned by name. The president of the USA in 2014 is Barack Obama in the same way that the capital is Washington. Nobody else can pardon the turkey. In essence this is the "defense of truth": even if the person did not do that particular act, they are the person who would do it.
If the Tuckerized political figure does something non-automatic and positive or reasonably noncontroversial, the author will usually refer to "the President" or "the Prime Minister", give a couple hints ("she" was enough for Margaret Thatcher back in the day) to confirm that the RL incumbent is meant, and let the reader nod. This is the same "nondefamation defense" that applies everywhere. It's nuanced - ex-VP Cheney might consider himself defamed (or at least insulted) if given a speech denouncing torturers and those who abet them as subhuman scum. Most politicians would be flattered.
When the RL figure (or others) could consider the depiction defamatory, walk wide! In a book such as "The Sum Of All Fears" (where Tom Clancy depicts an incompetent president and an evil National Security Advisor), a fictitious character is necessary, if only for good taste and publishability.