Our IS doesn't have rusty calipers either, and neither did my 3RX. My 4RX doesn't at the moment but its too new to really tell.
I don't think its the way they made but whats been used to clean the wheels, and how long calipers have been left without cleaning. Commonly, when people wash their cars, they miss out the calipers or they use strong acidic products which will cause this kind of rust on calipers.
The new Mercs have horrible centre caps, they need changing really often! So much so that my Father in law has got cheap ebay ones which look the same - yet they are much better at resisting corrosion. VW Aluminium ones tend to do the same. Mercs wheels are notoriously bad for corrosion too.