So what is different about Greenwald?
The news organizations he works with are different. Rather than publishing in the Washington Post or the New York Times, institutions that have particular, unique, and often cozy relationships with America's ruling class, he started out with a personal blog, later moved to Salon.com, started publishing stories sourced to Snowden at The Guardian's U.S. edition, and has worked with the foreign press.
His approach to journalism is different. Rather than trying (or purporting) to be objective, he is transparent about his opinions and explicitly argues for their validity. He criticizes fellow journalists for being insufficiently adversarial. Unlike most mainstream-media reporters, he voices contempt for certain American officials. And when he believes that they have broken the law, he doesn't shy away from urging that they be prosecuted and imprisoned for their crimes. It is no accident that there is no love lost for him in the national-security state.
Because he doesn't write for the outlets that the government considers most legitimate, because he is outspoken with highly polarizing opinions, because he can be abrasive, and frankly, because he's a gay man who lives in a foreign country, U.S. officials and some journalists correctly sense that they can get away with trying to delegitimize Greenwald in a way that would spark a backlash if they attempted it on Gellman or Woodward or any New York Times reporter.
That is unjust and dangerous. And journalists had better get wise to its most serious implications.
One reason to stand up for Greenwald is that there is no evidence suggesting that he's acted as anything other than a journalist on the Snowden story. Another reason to defend Greenwald is that whatever one thinks of him personally, or his politics, or his attacks on various mainstream-media figures over the years, prosecuting him as a criminal would set a precedent affecting all journalists.
The U.S. officials smearing Greenwald are well aware that a precedent would be set. They sense that, among the journalists reporting on the Snowden leaks, prosecuting Greenwald would cause the least backlash; they know that many journalists at the New York Times and the Washington Post would be reluctant to publicly state that, for all relevant purposes, what they do and what Greenwald does are the same; they know that Greenwald has made lots of enemies in the press, that he has no movement of loyal ideological hacks to rally around him, and that enough Americans think living abroad and being gay are suspicious to make a difference on the margins. So they chose Greenwald as their target in what may be a trial balloon for an effort to delegitimize national-security journalism.
There are precedents for smear campaigns of this sort.
The First Amendment and its guarantee of press freedom was created precisely to protect outspoken criticism of government officials, revelations of official lawbreaking, and material that changes the political debate by informing the public. There is no way to criminalize the work Greenwald has done on the NSA surveillance story without significantly undermining this core constitutional protection. And so, on this narrow question, every American has an interest in defending him.