So why not just print the rules for the Aux arc in the booklet that comes with these ships?
Small ships don't get booklets, they get reference cards, in this case a reference card for how Aux Arc's work.
I'm still confused why the Firespray didn't need a rule card for it's Aux arc, but these ships do.
Because the Firespray box was big enough to include a booklet that had the special rules for it, which included the rules for the aux arc.