Ultimately this isn't very relevant because right to free speech does not mean a right to web hosting infrastructure. But I agree with the Founders; regardless of whether it builds a more/less optimal society, government cannot and should not infringe upon our natural right to free speech. In particular, it protects us from jumping from your hypothesis (that unchecked free speech motivated a mass killing that would have not otherwise happened, which I give a 60-70% probability of being true) to an ironclad law forever removing some liberties from the citizenry.
The American Constitution doesn't argue anything about free speech, define it or make claims about its origin, or even refer to is as a right, much less a "natural" right. It only declares that Congress (explicitly, and exclusively, meaning not even inclusive of any other governing body or private entities) shall not pass laws that abridge "the freedom of speech, or of the press; or the right of the people peaceably to assemble."
GP's point is that the constitution serves not to grant the freedom of speech, but to restrict the government from curtailing it.
Yes, because that's what the constitution says, or rather because it doesn't say otherwise. All rights not granted the Federal government through the constitution devolve to the states, and further, to the people. The states and private enterprise are allowed to abridge the freedom of speech, just not the Federal government through Congress.
>GP's point is that the constitution serves not to grant the freedom of speech, but to restrict the government from curtailing it
GP's point was that the constitution made a philosophical argument about freedom of speech as a natural right that it does not, in fact, make. One can believe that freedom of speech is a natural right if one so chooses, but the only thing the constitution actually says about freedom of speech is that it's one of a list of freedoms Congress can't pass laws to abridge.