How so? Censorship is explicitly forbidden by the US constitution, and even so it happens (in this case, "in the name of national security" or whatever).
Remember the constitution only really, effectively, says whatever the current Supreme Court says it means. And really I don't think anyone want's the 100% literal 'shall make no law' interpretation of free speech; that would throw out any kind of labeling laws for starters, companies would have no government compulsion to accurately label drugs or food products for example.