I think part of the intuition arises from the fact that the unhappy—and seemingly insulted—president couldn't practicably leave while the speech was going on.

Recently, Michael Dorf wrote a perceptive column for this site on the role of the "captive audience" in First Amendment doctrine. As Dorf suggests, an audience may be deemed "captive," in free speech doctrine, when its attendance is either legally required (Dorf's example is teens' attendance at public school), or socially required (Dorf's example is family members' attendance at a funeral). Speakers' First Amendment rights to reach the ears of such audiences may be less than their rights to reach the ears of, say, passersby in the public square.

The Correspondents' Dinner was the rare instance where the president was himself a captive audience of one. By comparison, the president has the ability to stop taking questions at a press conference...