Even in that scenario you're likely preventing other people from having a meal. They can't assume people are going to come in, order dessert, and leave after 20-30 minutes which would open up additional reservation spots. Every reservation is assuming people are there for a meal. It would potentially open up a few standby tables, but most people don't even try to eat standby at a Disney restaurant so that's not really that helpful. It's more likely you'd end up with tables sitting empty until the next reservation -- in the event that people actually did only come in for 30 minutes, but it seems like people were doing it just to watch fireworks and likely staying much longer than that.
I don't think any individual person that's part of a larger party should be prevented from only ordering a dessert or an appetizer (or nothing at all), but if you have a group of 8 come in and all order desserts and nothing else they're hurting other guests and, as
@peter11435 said above, they're absolutely killing the waiter. They're not going to make nearly as much money as they would otherwise unless the people offer a huge tip far above and beyond 15-20%, which isn't going to happen the vast majority of the time.
I don't really have a problem with people going to a restaurant just to have dessert if it wasn't at Disney (or other similar locations). At Disney, though,
there already aren't enough restaurants at Disney to go around. When people are taking up spots simply for dessert, you're forcing other guests to eat a QS meal or go to a low quality TS restaurant. I don't think it's fair.