I took the Carolinian from Washington to Charlotte and back last month. Southbound the cafe car closed at Selma-Smithfield and reopened upon departure from Raleigh. Northbound, the cafe closed just prior to arrival at Cary and reopened after departure from Raleigh.
In both cases, it was announced that the crew was changing at Raleigh. The cafe counter closed to allow the attendant to take inventory, as has been stated above. Additionally, since new crew came on board, all tickets had to be rechecked and I assume that's why they wanted people back in their seats.
My biggest peeve was that on the northbound run, they did not announce ahead of time that the car was going to close. I was in the last car of the train, and was steps away from the cafe car when the conductor came on and said something about the cafe and closing. Anyway, when I got there, 30 seconds later, the cafe car attendant was not around, and the business class attendant, who was sitting in one of the booths, informed the collected masses (5 or 6 passengers) that the counter wouldn't reopen until after Raleigh. It's fine to close. But it would be better to give people 15 minutes' notice, so they don't walk through 4 cars just to essentially have the door slammed in their face.
Now, I don't know if this has anything to do with it, but the midday Piedmont service has not been running due to trackwork. That may have affected the crew change point. Or maybe not. I have no idea. It was my first trip on the Carolinian.