That's because many of the sidhe don't bother learning about the "lesser" fae.  Essus was uncommon in wanting his daughter to have a well-rounded education in all aspects of the fae -- but then again, she had a brownie for a grandmother, so maybe it's not all that uncommon.<br/><br/>There are probably sidhe who know about certain customs of the other courts, if they don't care too much about how dealing with the lesser courts make them appear in the eyes of their sidhe peers.  Andais certainly knows about a lot of the customs, and probably Taranis too.   Remember, it's only arrogance that keeps sidhe in the dark about the other courts' customs.  It's not like the demifey and the goblins try to hide their customs and laws.