No, see, I understand all of that. But the way it was described, anyone can apply for a Letter of Marque and maintain it as long as they fill their pirating quota. It's the command position on the pirate ships that are hereditary, not the Letters (something like Rogue Trader vessels, now that I think about it), and the Bentenmaru is a big deal because it's an ancient and venerable ship (essentially the equivalent to a long-standing house of nobility), not because it has a letter of marque. And my point still stands: there is absolutely no reason present for the nebulous galactic empire to tolerate the continued existence of piracy, much less officially sanction it. The faction that employed pirates, and the faction that they were used against, are both explicitly no longer extant entities, and it has been so long that most people don't even know that pirates are still around. Except, that is, if they're keeping them around as tourist attractions, which seems to be the case.
And hey, at least with Girls und Panzer you get plenty of tank battles. I think there were maybe two episodes (barring the recap/Meet the Cast ones) that didn't have at least part of their time dedicated to armored combat. Meanwhile I've watched up to episode 9 of Moretsu and there has yet to be a single real battle. A long period of waiting followed by a brief, ineffective exchange of fire, two touristy boarding attempts (with one staged swordfight), and one short period of trading warning shots as part of the show for the tourists. Meh.