Or put another way, free speech is not what you think it is. Howard Zinn explains here. Long but interesting.
In sum legally the first amendment has been interpreted via the doctrine of "No Prior Restraint". You can't be prevented from saying something, but your words can be judged illegal after the fact. Note for example the passage of the "Alien and Sedition Act" just 7 years after the passage of the first amendment, which made it a crime to write or say anything that would bring in to "contempt or disrepute" the US government, Congress, the President, etc.
The courts have sustained this interpretation into the present day. People distributing leaflets opposing the Vietnam War were arrested, and those arrests were upheld by the courts. That was 1972. In 1986 the courts barred the PLO's UN observer from entering the US to participate in a debate saying it would harm the US policy of not recognizing the PLO.
I'm reminded of this for 2 reasons. First a hilarious Onion article which is a profanity laced Supreme Court ruling affirming free speech in a recent case. And Bob Dutko's frequent arguments regarding separation of church and state. For the Onion free speech is an obvious right. For Dutko the fact that Congress immediately made religious gestures after the passage of the first amendment is proof that they didn't intend separation of church and state the way atheists do.
I think Dutko and the Onion are partly right, but you need to keep in mind an important caveat. The Constitution is whatever those presently in power say it is.