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.

There's only one law, and it is pretty specific:

"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.