A couple of points from a shop owner:
Fluid exchange is the correct procedure, and since it was a warranty job that's what GM is paying for, no more. Assuming the dealer used a modern exchange machine it is better than dropping the pan. You simply connect inline to the trans cooler and do a 1:1 exchange of fluid, using the transmission to pump the fluid through all gears. It gets ALL of the old fluid out of the unit. Whereas dropping the pan and replacing a filter only replaces what is in the pan or about 40%-50% new fluid.
"Flushes" are a thing of the past. The "flush" terminology earned a bad rap because the older "hot flush" equipment used machine pressure to blast fluid (and sometimes solvent cleaners) back through the trans, which could cause issues.
Another benefit of the exchange is time. It books about 1.0 hours, where a pan drop can book for about 2.0 hours or more. On a pan drop getting the fluid level perfect is a bit of a process (warm-up time, computer temp verfiy, racking a second time for level check etc). It can be an real PITA on other makes with insane procedures. With the exchange machine it's 1:1 in and out, none of that is required. Since labor time equals cost the price to you is much cheaper for the exchange. If you request a pan drop/filter you're going to be paying $labor x 2.0hrs+ and fluid and filter vs $labor x 1.0 plus fluid only (or a fixed menu price in many cases).
Also modern transmission filters are more of a screen and while they can benefit from service at higher mileage they are not the same as paper element filters we grew up servicing in TH350's etc.