The
Articles of War are specifically rules for the military. The original Articles were written for the Royal Navy, the British Army got their own version, and other navies and armies probably had something similar.
Those who don't get a LoM or join the navy don't need to know military codes of conduct. They just need to know that if they attack anyone who didn't attack them first then they're pirates. So there's no need for them to get the book from a governor. A merchant's main concern about relations is who is at war with his country, therefore whose ports and ships he needs to avoid.
And pirate characters, whether by original choice or by becoming pirates due to misconduct, don't much care about rules anyway.
That said, the book could perhaps be available for sale. Either make it rare because some retired officer sold his copy, or make it common because the government wants people to know how its navy is expected to behave and so puts out plenty of copies for sale.