A criminal act
No matter what the motive, assault is a criminal act. In my book, willfully violating a criminal law that has been constitutionally enacted by the will of the American people can be described as "unamerican" no matter what the motive may be. Had Mr. Barton initiated the assault because someone verbally complained about the message on his t-shirt, that act of assault could be described as unamerican. In this case, though, it appears the other party, for whatever reason, is the criminal party. One hopes that Mr. Barton repairs quickly and that the perpetrator is eventually caught and properly charged.