Symbolism, Imagery, Allegory
There's no way Calvino was going to let you out of this one without throwing some political commentary in. Most of the beef comes in later on in the book, after you (the character) have flown to Ataguitania and become implicated in the exhausting network of revolutionaries, counterrevolutionaries, real police, fake police… the list goes on.
In a chat with Arkadian Porphyrich, a high official in the country of Ircania, you learn that the dictatorships operate almost entirely on fictional premises, since they intentionally import the same books they ban. Wait, what? Why would they import the books that they prohibit in their country? Porphyrich explains:
every regime, even the most authoritarian, survives in a situation of unstable equilibrium, whereby it needs to justify constantly the existence of its repressive apparatus, therefore of something to repress. (19.7)
In other words, governments create their own enemies in order to have something to show their authority over. Sneaky.
Earlier in the book, the letters of Ermes Marana explain how the translator has been able to strike a deal with an African dictator named President Butamatari; this deal will force Silas Flannery to write a novel praising the dictator's efforts to absorb the provinces surrounding the dictator's country. Throughout If on a winter's night a traveler, there's a deep connection between censorship, propaganda, and government authority. Although as Calvino points out, it's not always as simple a relationship as we might think.