My guess is that this is common to all cruise ships because there’s a reason for it. The main restaurants are churning out hundreds of meals an hour. It’s hard to do that and have each one be top notch. The upcharge restaurants probably have their own kitchens, or their own part of the main kitchen, and are only serving tens of meals an hour: so they can focus much more on quality.
I’m very pleased that I managed to get a reservation at Palo on the Magic. I booked brunch so that I don’t miss out on the standard three rotational restaurants. Many critics — not least Touring Plans itself — say that brunch is better than dinner.