The pirate captain was quite self aware when he explained to Alexander why they could not just sail onto any port and dump their ill gotten goods there.
It tended to cause a lot of trouble.
Plus they themselves tended to cause a lot of trouble when on land, harassing businesses and being a menace to women.
It was thus little wonder their sights were a difficult thing for the local lord to swallow.
And this was not to mention the far bigger problem of trading with pirates.
Doing it so openly was not just frowned upon, it was massively problematic.
And this was because of how the lord used these pirates.
They let these 'detectable' proxies attack other cargo ships instead of using their own navy as a way to give themselves a veil of deniability, in order to not directly go into a war with the other party.
But this did not really work if you were actively buying their plunder.