Years ago Jane Fonda sponsored a school in Hollywood to train up and coming actors on the proper way to do things like conduct interviews, dress, speak and so forth. A healthy component of the class work was the proper political stance to take on various issues. Evidently up and coming actors signal their intent to be part of the system by going public on their political stances. I am quite positive the Matt was one of the pupils as was his sidekick Ben Afleck.
To paraphrase the Blond Bomber, "Matt, shut up and act".