That's a pretty fundamental structural failure that should not happen.
I don't just mean it shouldn't happen to such a new car, or that it shouldn't happen to a car with less than 40,000 miles, I mean that in 2120 when someone stumbles across the rusted carcass of your Pontiac with a scattering of faded plastic body accents buried in the ground around its perimeter, with the body sheet metal crumbled into reddish flakes and powder and the engine block a solid rusted mass of iron with fuzzy corroded aluminum bits attached, the engine cradle should still be firmly attached to what is left of the body sub structure.
If the dealership doesn't cover it, climb the corporate ladder until you get someone who takes appropriate action. But first, contact NHTSA and file a safety complaint--if this is a structural problem, it might warrant a recall.
In or out of warranty, this is a failure that GM should cover 100%.
Jim