[Fluorescence]

This doesn't seem to be true. Implications of Government speech are not taken lightly and I can see examples of "informal censorship" including implied threats of legal action being classed as prior restraint. Pretty moot given today's US legal environment though.

e.g.

https://en.wikipedia.org/wiki/Bantam_Books,_Inc._v._Sullivan

"Justice William J. Brennan Jr. delivered the majority opinion that stated that the actions of the Rhode Island commission to Encourage Morality in Youth were unconstitutional due to their actions violating the First Amendment by placing a prior restraint on free speech. Justice Brennan's opinion stated that the commission's practice of notifying book distributors and retailers about "objectionable" publications, combined with implied threats of legal action, effectively made a system of informal censorship."