At the Washington Post Eugene Volokh helpfully summarized some of the other legal considerations governing heckling. The short version is that an occasional “boo” is no more illegal than applause is but organizing people to disrupt a political rally or meeting is illegal.
My gripe on this issue has been the reaction of the Chicago Police and Rahm Emanuel. Contrary to the mayor’s assertion, Chicago did not stand up for freedom of speech but quite the opposite. It stood up for those who were abusing their freedom to suppress the free speech of others.
Supporting freedom of speech is most difficult when you dislike the speaker or believe that his or her ideas are wrong or pernicious. As has been said, these are the times that try men’s souls.