Assuming the article is correct (and the video seems to back that up), I wouldn't even consider this protesting. Yelling over Clinton's speech, holding up signs, harassing people going to the speech, throwing shoes -- these are all protests. Quietly standing with your back to the speaker? Not a chance.
I would call it a protest, a silent one, and with some degree of effect as well especially if cameras ever showed the audience. However, such prompt removal for such a passive/peaceful gesture is troubling.
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
They could have asked him to leave. Then demanded he leave. Then arrested him if he refused to leave. That would have been legal.
Unfortunately, the amendment didn't specify where you can assemble and petition. Government in recent years has been making sure that's a "free speech zone" that's quite a distance away.
They removed him because they don't allow you to petition the government, you know, within earshot.