Rob Kerr/AFP/Getty Images
The photo (source: The Guardian), from January 4, shows Ammon Bundy speaking at the militants' daily press conference. I have no idea who the guy is on Bundy's right, garbed in the green kufiya.
As you can see from the photos below, from January 8, taken by Rick Bowmer for AP (found here), sporting kufiyas seems to be pretty routine for these guys.
I suppose there are at least a couple of sources here, and I'm pretty sure that donning kufiyas for these guys has nothing at all to do with Palestinian solidarity. First, kufiyas were and no doubt still are worn routinely by US soldiers in Iraq and Afghanistan, going back to 2003. You can find "military" ones easily with a google search, although they are more commonly called shemaghs in such circles, it seems. I have a fire retardant (!) khaki kufiya brought me by someone who served in Iraq, and I think it is military issue. I've posted about military kufiyas, which I've described as examples of "tough guy" kufiyas, here and here.
Second, over the past decade or so, there have been several occasions where bad-ass tough guys in Hollywood movies are shown in kufiyas. My favorite is John Travolta in From Paris with Love, but there are lots of examples. Here's a few, but you can find many more in this blog.
Finally, kufiyas have been showing up in gun catalogs. For instance, this photo, from the Griffon Industries 2011 calendar. The woman (Miss June) in the kufiya and camo bikini top is being used to market the gun. (There are more items around like this, and I need to do more searching, and posting.)
My guess is that the armed militants in Oregon are wearing kufiyas, inspired or influenced by the three sorts of uses described above.