After a long discussion of the breadth of the speech restriction involved in the case, Chief Justice Roberts’ majority in United States v. Stevens goes on:
Not to worry, the Government says: The Executive Branch construes §48 to reach only “extreme” cruelty, and it “neither has brought nor will bring a prosecution for anything less,” The Government hits this theme hard, invoking its prosecutorial discretion several times. But the First Amendment protects against the Government; it does not leave us at the mercy of noblesse oblige. We would not uphold an unconstitutional statute merely because the Government promised to use it responsibly.
This prosecution is itself evidence of the danger in putting faith in government representations of prosecutorial restraint. When this legislation was enacted, the Executive Branch announced that it would interpret §48 as covering only depictions “of wanton cruelty to animals designed to appeal to a prurient interest in sex.” No one suggests that the videos in this case [which involve dogfights and dogs attacking other animals] fit that description. The Government’s assurance that it will apply §48 far more restrictively than its language provides is pertinent only as an implicit acknowledgment of the potential constitutional problems with a more natural reading.
UPDATE: Prof. Doug Berman (Sentencing Law and Policy) asks whether this passage foreshadows trouble for the federal “honest services” statute, which is the subject of some pending Supreme Court cases.