The Founding Fathers had quite a bit to say about free speech, and the reasons for it. Some of them later regretted it, but it was a bit late then. To understand it, however, you will need to look at the philosophies of the Founding Fathers, and the philosophies that developed both before and after as a consequence.
See also how suppression of free speech was used in both Britain and Europe at the time, the effects of inconvenient questions on power structures in Europe and elsewhere (such as the traditional nailing of theological questions to the door of the church), and how telling the truth could make your words famous or so dangerous that it was impossible to get your hands on, or, in rare cases, both (Niccolo Macchiavelli's "The Prince", for example).
Think also on who was granted the right of free speech, against whom it was directed, and see if that can spark some ideas on the reasons for doing so, and possibly the times and places where it is not valid.