I really hate this rather common misunderstanding, and abuse of, the first amendment rights.
When you're on "someone else's premises", as you almost always are online, you do indeed waive your rights to complete freedom of speech. By registering and posting here, you're explicitly agreeing to abide by the rules. Don't like it, go somewhere else with rules you like, or start your own. Really, no different than a brick and mortar store posting rules like "no soliciting" on the door. Rules of the house always apply, unless they explicitly contravene your constitutional rights - rules like this do not, you're still free to say whatever you want, just somewhere else.