I'm a journalist, and like all journalists I'm sometimes lazy and sometimes unfair. I'd certainly benefit enormously if the law were changed as you suggest: to let me off the hook for the most scurrilous accusations, provided only that I prefaced the story with the phrase "It is rumored that." Wouldn't journalism be fun then!
But nonjournalists have rights too, and among them is the right to reputation--which courts have always recognized as a species of private property, and thus every bit as worthy of constitutional protection as the right to publish. No, I don't want to see the abuses of present-day tort law inflicted on newspapers. But it's silly to say that the only alternatives are either $35 million for a too-hot cup of coffee or total legal immunity for McDonald's.
I completely agree with you that rumors are sometimes news, and that newspapers should often inform their readers of them. But there are proper and improper ways to do it. If Republicans really were circulating false stories about Sidney Blumenthal to discredit him, yes, sure, that's news. But a reporter thinking of printing that news should also ascertain as best he can whether the underlying story is true or not. That's part of the story about a rumor campaign too, isn't it? So far we've been using examples of Republican rumor-mongering. But consider one that involved Democrats: When the 1988 Dukakis campaign spread rumors that presidential candidate Joe Biden had plagiarized a speech from Neil Kinnock, wasn't it important for reporters to see the videotapes before they published the charges? Isn't it true that the story has one meaning if the rumors being spread about Biden are true, and quite another if they are false? Doesn't the public deserve to know which is the case?
As a journalist, one hears all sorts of stories, many of them very titillating. It's one's job to check them before printing them. If a journalist fails to do that, he or she can inflict immense harm on completely innocent people. Journalists cannot escape their responsibility to make reasonable efforts to ascertain the truth of what they say by pointing to some source and saying, "I only printed what I was told." That's what the courts have traditionally held, and it makes sense to me.
You ask why you cannot go into business as a journalist pro se: develop sources, listen to what they have to say, and decide for yourself what to believe. Of course you can. But Matt Drudge (and those who will follow him) is asking for more than that--he's asking for the right to rebroadcast the stories he hears, and to an audience that potentially includes every English-speaking person with a modem. American law doesn't ask him to verify his story beyond a doubt. If he's writing about a public figure, it doesn't even ask him to do a particularly good job of verifying it. All it asks for is a minimum good-faith effort. Is that really too much?
Blumenthal, as you say, is a man with considerable power in the media. He succeeded in scotching this story, and along the way did damage to his political opponents. Lucky for him. But not every person about whom damaging stories are told by a journalist will possess Blumenthal's clout. What are those poor devils supposed to do if journalists are given freedom to defame with impunity, providing only that they take care to invoke the magic words "I heard a rumor that ..."?