Walk-ups pay just as much for their food, and are just as profitable, as someone who made a reservation a month prior. They are trying to cut back on scheduling, but they are "leaving dollars on the table to save pennies".
You never want to have a completely sold-out,100% full restaurant (hotel, train, plane, etc.). If there are but a couple of seats empty, you know you have sold all the tickets/meals you possibly could without turning business away. In a sold-out condition, there is no way to tell just how much (potential) business you lost. By cutting staffing to the level of advance reservations, if Disney will be unable to accommodate last minute (day of) customers, they are guaranteed to be turning people away to leave empty tables.
I've never seen a doctor or dentist do that, but if they pulled such a stunt I think I'd find a new doctor. Hotels do charge you, but that's different for one reason - they hold a CC reserved room all night waiting for you and thus cannot sell it to someone else. You have bought the room whether you use it or not. That's not true of a restaurant, which merely need to ensure they don't get stuck with large numbers of no-shows. To take the hotel example a bit further, hotels often permit cancellation up to 6 p.m. the day of arrival. Providing 24 hours notice to a restaurant is ludicrous.