This is the way all cruise advertising is done (resort ads or almost any other type of travel, too for that matter). If you look at any magazine or TV ad, put out by Disney, Carnival, Hilton, Marriott or whoever, they all say "rates starting at $X.00" and give the lowest price. But if you'll notice, somewhere there is always fine print somewhere that reads "subject to availability, black out dates, limited number of rooms available, etc. etc." It's not bait and switch. Those rates are or were available for some date, somewhere -- it's just not likely to be dates or categories or rooms that most people will want and there aren't great quantities of them to start with. When they're gone, they're gone but the ads can say "starting at..." and be covered, because those were or are legitimate rate offers that people can get for certain times, categories, etc. And even if they are currently sold out of those rates, they can leave it up because more rooms might be made available at that rate, new dates might open up for those rates, people might cancel, etc. Since rates are constantly in flux in travel, the lowest rate can always be advertised, because it might be or become available at any time.
A true bait and switch is when an offer is made and then you get to the store and they are "out" of the offer (in a true bait and switch scam they never had the item to start with) and then the salesclerk tries to aggressively talk you into the higher priced item. At DU (or any other reputable agency) they aren't going to try to hog tie you into a different rate. If you don't want to cruise at the rates they're offering, you can pick another date, category, etc. or just not go. These rates are or were available, just maybe sold out or not available for your dates or desired category.