Yes, Viggo's shirt invited comment, but it's still Rose's show and as such Rose should know how to control the subject and keep it on topic. The topic was LOTR, not Viggo Mortensen's response to the U.S. Government's political stance against the Middle East.
Had Rose acted as a proper host, he would have acknowledged it, and then said, "While this show certainly deals with subject matter of this nature on a regular basis, I think this week we can break away from that. I would love to have you back in the future, Mr. Mortensen, to discuss your views, and we can certainly incorporate more discussion of this relation to LOTR as you perceive it. But for now, let's focus on more cheerful thoughts and the movie..."
Instead, Rose chose to drive forth and make it a one-on-one with Viggo, really pushing both Peter Jackson and Elijah Wood to the side. Viggo's shirt made enough of a statement to invite him back for a more direct discussion later, and if Viggo had truly wanted to discuss it further, no doubt he would have accepted the invitation.
Such actions probably would have produced both a fun show for us to watch about them promoting the movie and other things, and also could have given us something more to look forward to, should Mortensen have agreed to come back to discuss his views with Rose on a later program.
That's all I'm getting at.