This was something the builder just did (I wouldn't even call it convenience/cost, for that matter).
We start with 210.52(B)(1):
(1) Receptacle Outlets Served. In the kitchen, pantry,
breakfast room, dining room, or similar area of a dwelling
unit, the two or more 20-ampere small-appliance branch
circuits required by 210.11(C)(1) shall serve all wall and
floor receptacle outlets covered by 210.52(A), all counter-
top outlets covered by 210.52(C), and receptacle outlets for
(1) Small-Appliance Branch Circuits. In addition to the
number of branch circuits required by other parts of this
section, two or more 20-ampere small-appliance branch circuits shall be provided for all receptacle outlets specified by
Between these two sections, we can establish that the dining room outlets are part of the small-appliance branch circuits, not the general lighting load.
However, 210.8(A) point 6:
(A) Dwelling Units. All 125-volt, single-phase, 15- and
20-ampere receptacles installed in the locations specified in
210.8(A)(1) through (10) shall have ground-fault circuit-
interrupter protection for personnel.
(6) Kitchens - where the receptacles are installed to
serve the countertop surfaces
requires only that the receptacles installed to serve the countertop surfaces in the kitchen be GFCI protected. This means that no other receptacles on the small appliance branch circuits require GFCI protection under the 2014 NEC, regardless if they are located in the kitchen proper or in a separate dining room.