Princess's stated policy: "The evening dress code applies throughout the ship, except designated casual areas, after 6:00 PM." The only "designated casual area" is the buffet area (called the "Horizon Court" on all ships that offer "Anytime Dining").
How do they get those "casual" folks back to their cabins? Transporters?
That's the problem they've created, by creating a casual dress public area for evening dining. They honestly don't expect those people to go to dinner and then directly back to their cabins... so their policy is uneforceable, and they don't bother trying.