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Commercial, as in "relating to commerce." I've never listened to Imus, and never will, which makes this an interesting decision by MSNBC as what he said probably would not cause any significant percentage of his audience to change the channel.
What it does signify is that MSNBC is perceiving a significant amount of collateral damage to markets outside of Imus' sphere of influence. This is encouraging as hopefully this perception will lead to less commercial sponsorship of ignorant and intolerant speech (including demeaning rap music lyrics). But let's not kid ourselves: no meaningful change will happen if the execs think there is a market and money to be made.
As to the free speech "issues," Joan is right: it only protects the government intruding on your right to speak freely. It doesn't extend to someone's perceived guarantee to lucrative media deals. I'm not privy to the details, but I would imagine that if he had signed a contract, he breached one of its provisions. Otherwise, he's toast whenever one of his clients feels he's more of a liability than an asset.