GM lawyers are smart. Warranty claim reduction incentives drive them to use skillfull tactics.
If they rated the trannies for more power then they might have to pay out a claim for a fried unit, but if they rate it low, and they save money there too on more costly parts like LSD'*, then if you modify the engine for more power and you & I know that means you'll be driving harder than normal therefore far more likely to have a warranty claim, and if you blow that trani, GM can deny the claim on the basis of the modification exceeding the rating of the equipment.
Maybe I'm just delusional but you never know with these large corporations. Every penny counts.