This is a very common practice in my neck of the woods. Always assume prefinished maple interiors unless you specifically noted otherwise. Those who have built entire custom kitchens will understand...
You can buy prefinished maple plywood in sheets and use it to make the cases of all of the cabinetry. This gives a durable, cost effective, factory finish for the interior of the cases. This way the cabinetmaker only has to finish the face frames, doors and drawer fronts; meaning you save a lot of money by not having the interiors hand painted or sprayed. That adds up to quite a lot of money for a whole set of cabinets.
Same goes for drawer boxes, factory prefinished maple with factory dovetailed joints are the standard.

Steve, thank you, appreciate the perspective. By this same, the underside of the upper cabinets are left with prefinished maple, not painted (With the exception of one which they did paint the bottom). The piece numbers or builder notes in sharpie pen that they wrote are visible. I asked my GC what he thought as I also expected cabinet maker would paint the bottoms and he said he figures the cabinet fits assumed the bottoms of upper cabinets would be out of sight and therefore didn't need painting. While they aren't "easily" visible, it doesn't seem right not to paint the entire exterior of the upper cabinets in my opinion. Appreciate your thoughts. Attached image shows one of the uppers, it is upside down on the floor so you can see the underside. In this shot you can see they painted part of it white and not the rest. None of the rest of the undersides of uppers is painted.

The exterior of cabinet boxes are unfinished unless you specify otherwise. People stack cabinets, install light rail, add finished bottoms and sides. There is no point finishing the exterior unless it is going to show. I realize you knew it was going to show, but unless you specify finished bottom/ ends you get unfinished.