McClellan is just a former press secretary, with his fair share of inside stories (true or not), grudges (justified or otherwise) and financial needs. He may have children to put through college, or a dignified retirement for himself and his wife to hope for, and he probably cannot expect some lucrative job to fur-trim his later days. He has had his moment in the sun, and the warmth is only going to decline from now on.
So I have a certain amount of sympathy for the squeeze in which McClellan found himself, and for the temptation he felt when his editors suggested (as they apparently did) that a bigger book advance would be justifiable if he could just think of some piece of raw meat to toss to the political and journalistic dogs who are always out there, ready to chew on the incumbent president of the United States.
As a matter of fact, the real surprise is how minimally discreditable Bush's alleged false "disclosures" were. Bush's central charge against Saddam Hussein was that he had nuclear weapons -- a belief Hussein worked heroically to spread, with such success that it was believed by all of the world's major intelligence agencies (including ours). If Hussein had indeed had them, and used them, Bush's critics would now be trying to impeach him for not knowing this and taking pre-emptive action to disarm the Iraqi regime.
Instead, Rich is reduced to complaining that "Americans don't like being lied to by their leaders" -- as if he has demonstrated that Bush knew the charges were false. But it isn't their leaders who are lying to the American people. It's the propagandists like Rich.