A Leftist writer below argues that it is and he actually has a fairly good case in terms of existing legal precedents. SCOTUS has drawn a rather fine line over what sort of advocacy of violence is permissible and I think they have got it pretty right. They say that advocating violence in general terms is OK but not in specific terms.
So: "The capitalist system must be crushed by armed revolution" is OK -- being essentially an expression of opinion or belief -- but "Meet at my place at 10am tomorrow with hammers and we will go downtown and smash the windows of all the banks" is not OK.
And the sort of Islamic advocacy that our Leftist defends below would seem to fall into the protected "general" category rather than the banned "specific" category.
On the other hand, the terrorists by their own admission are at war with us and speech restrictions during wartime are generally defended in the name of protecting the population. "Loose lips sink ships" was a famous government slogan in WWII. So safety trumps free speech. And I think that could be argued in respect of the cases below
Over the past several years, the Justice Department has increasingly attempted to criminalize what is clearly protected political speech by prosecuting numerous individuals (Muslims, needless to say) for disseminating political views the government dislikes or considers threatening. The latest episode emerged on Friday, when the FBI announced the arrest and indictment of Jubair Ahmad, a 24-year-old Pakistani legal resident living in Virginia, charged with "providing material support" to a designated Terrorist organization (Lashkar-e-Tayyiba (LeT)).
What is the "material support" he allegedly gave? He produced and uploaded a 5-minute video to YouTube featuring photographs of U.S. abuses in Abu Ghraib, video of armored trucks exploding after being hit by IEDs, prayer messages about "jihad" from LeT's leader, and -- according to the FBI's Affidavit -- "a number of terrorist logos." That, in turn, led the FBI agent who signed the affidavit to assert that "based on [his] training and experience, it is evident that the video . . . is designed as propaganda to develop support for LeT and to recruit jihadists to LeT."