It's a fallacy that there's a morally or legally significant difference between what Robert Halderman stands accused of doing, and what agents and lawyers do all the time. Halderman had a marketable story to sell. Just because most agents and lawyers wouldn't go to David Letterman's house doesn't mean that going to Letterman's house is a criminal act. It's not unlawful to sell a story without using an agent or a lawyer, and making a proposal to someone who might be embarrassed by the dissemination of a story isn't prohibited.
If the threat Halderman allegedly made had been made by telephone or at lunch (or however agents/lawyers conduct business), the meaning and effect would be the same as the pitch Halderman apparently made. No matter how you slice it, the message would be that embarrassing information about Letterman will be disclosed to the public unless a $2 million payment is made.
Halderman's rep to Letterman's rep: "If your client doesn't buy the rights to my client's story for $2 million by Friday, we'll offer it for sale on the open market, which could cause your client and his family great distress."
Why should it be legal for an agent or lawyer to use such negotiating tactics, but not for someone without representation to do so?