Bill's right. Imus broke no law. He broke a rule. He probably didn't even break an explicit rule, but he certainly broke an implicit rule or two. And one of the rules he broke is one that governs the relationships between employers and employees everywhere:

If you say or do something that drives away customers, you are subject to termination.

It has nothing to do with freedom of speech. The First Amendment applies to government, and only to government. It does not apply to the workplace.