Can a cruise ship force you to dress up?

Is it legal for a cruise ship to force you to dress up for evening dinner? This is going to be a 7-day/6-night cruise and there are going to be at least 2 formal nights, if not 3. Of the 15 restaurants on the ship, there are 4 of them that require formal attire. The itinerary says that you must have formal attire in the main dining room as well as the other 3 dining places. Can they force you to dress up?