I have an MS in mechanical engineering focused in materials and I wrote a few papers on 3d printing of stainless tool steels. I've read further down the thread and you are completely wrong.
It's rusting because it's a low grade stainless. Its low grade because stainless steel work hardens when bent or machined. Lower grades have less issues with work hardening. They made this choice to save on tooling costs I assume.
Work hardening is when you permanently deform a material it gets more brittle. Stainless is an unbelievably stupid choice for a vehicle body because it needs to be formed into complex shapes which require lots of deformation. It's why SS cars feature mostly large flat panels and also partially why we have been moving towards aluminum body panels.