Why is blackmail illegal?

So, you’ve probably seen the story about David Letterman being blackmailled over his affairs. This forms the basis of a thought provoking piece in the New Yorker asking why blackmail is illegal:

    A final call went out to Walter Block, the libertarian economist. Block believes that blackmail, like smoking, is “yucky” but should be legal. “He only threatened to be a gossip—maybe a screenwriter,” he said of Halderman. “Screenwriting and gossiping are legal. If it’s legal to do it, it should be legal to threaten to do it.”