In the car, disintigratged half was facing down. Car Owned since new, gets a hosing underneath to wash off salt ASAP in winter.

Such an interesting photo and subject.
My guess is that the rusted part is more base than the other part.
or the rusted part was on a more noble metal.
Electrolysis is what eats up metals. The base metal is the donor.

Have you ever seen a unibody (welded car body) rust out at the welds
between panels? How about it's the wrong welding rod for the sheetmetal,
setting up essentially a series of semiconductors at the junction of the unlike metals- electrolysis - destroying car bodies. Makes you wonder about using
stainless near ordinary steel.

Would GM, Ford and the others do that on purpose?

Well I just purchased both clamp halves new from volvo and same part#, probably regular steel painted black. I've developed a few theories, 1. tiny undetectible gas leak acted as paint remover on the bottom part. #2, tiny drain hole in lower part clogged, allowing some water collection. #3, somehow that lower part was swapped from another car during some service screw up.

It's a mystery! Doesn't seem that each half is the same metal.