The First Amendment of the Constitution states that:
“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.”
Yelling “Fire!” in a crowded theatre is one example of a situation in which the government found a compelling argument as to why freedom of speech should be restricted. While similar cases define the circumstances under which freedom of speech can be regulated by the government, it does not follow that anything not explicitly restricted is protected speech since the First Amendment only applies to government action.
For example, on a smaller scale, the speech of employees of private companies is not protected by the First Amendment with respect to consequences created by their employers. Thus, if an employee were to curse at his boss, his boss would be fully within his rights to fire him.
Going back to Don Imus, his being fired by CBS and MSNBC was not, in fact, a freedom of speech issue. Don Imus was fired by his employers after using racial epithets primarily because major advertisers were withdrawing their support, not because of the language actually used. However, even if it were about the language used, since CBS and MSNBC are private companies, the First Amendment does not apply to their actions.