Here’s how the space piracy business apparently works:

• The Galactic Empire wants to preserve the cultures of its subject planets. Thus the pirates are tolerated much as we might tolerate a large, colorful and slightly dangerous endangered animal.
• The Empire is rich enough that losses from piracy can easily be covered by insurance.
• There are implicit rules that everyone plays by. The planets that allow piracy use the Letter of Marque system to keep the number of pirates small enough that they are no more than a minor nuisance.
• The pirates co-operate by wearing the silly costumes and hoking it up so that the victims feel more entertained than upset. The unwritten rules of engagement ensure that nobody gets killed as long as everyone plays along.

Also it is now clear that this civilization does indeed have artificial gravity. It just isn’t used in the short-range ships that fly around the Tau Ceti system. That still doesn’t explain why the girls would wear skirts in those ships.