Question for the PS Guru's - Digital Grey Card Action Help!

Well, that is NOT the computer determining, but the user setting a preset... that will not take into account the contents of the picture it is used on.

And I also don't think you picked a good example of your theory. The corrected version of your picture has massive magenta-cast, especially in the area's that should be blue originally. And that's logical, the denim shirt's color is very close to the cyan color cast that is in the picture, so in fact it isn't contaminated in color. The PC doesn't know that, and will try to correct the color of the shirt, mixing it with magenta and resulting in purple.

This effect is also often seen in corrections of underwater pics, making blue objects and shadows purple or reddish. Nothing else to do than go in there and make targeted adjustments instead of global ones. And that is something you cannot ask the computer to do.