Here is how I see it. The Constitution does enumerate certain rights granted to us by God ("We are endowed by our Creator with certain inalienable rights...", Declaration of Independence), and freedom of speech certainly is one of them. But...
With any freedom comes responsibility. Each and every one of us has a responsibility to preserve and protect our freedoms, but to never ABUSE them. Every freedom has its limitations - limits determined by sanity and reason. For example, it is illegal to yell "FIRE" in a crowded theater. By the same token, inviting a madman and terrorist to openly speak to impressionable minds may not be the wisest, sanest or most reasonable example of exercising free speech. It strikes me to be a lot like yelling FIRE in a crowded theater.
Certainly, his speaking engagement has helped to prove to us all that he is, indeed, a monster, so in that respect, it is not a total loss. But at what cost? How many impressionable kids were taken in by this demon? How many "Adam Gadahn's", the American traitor and spokesman for bin Laden, did this exposure start to create? How many innocent people will eventually suffer because of our unreasonable tolerance for such people?
Here's the short take. I know evil when I see it. Ahmadinijad is pure, unadulterated evil in its raw form. The only platform he should be given is the one on the gallows. He should not be allowed to address Americans directly; he should not be allowed to address the UN; he should not be allowed to enter our country.
As the Bible says, "If thine eye offend thee, pluck it out". If you do not remove an infected part, the infection will spread.
Terrorism is an infection, and this madman is the head of the snake. I do not think we should abuse our rights to the point where we invite the snake into our home, then allow - nay, encourage - the snake to strike us dead.
Dead folks have no use for Constitutional rights.