I just had a look at Tirerack, and I'm confused too....ahh, spoke with the techs, and looking at the Tirerack site I think I've figured it out. There are 15"
alloy rims, but no steel rims. This is because the centre of the steel rims have a bowl shape that is welded to the outer rim further back and won't clear the brakes. The centre of the alloys, being one piece, is flat, leaving enough room to accomodate the rotors on the inside of the wheel. And MINI don't produce an alloy in 15" with an offset that clears the 11.5" rotors on the 'S', hence only the 16 " alloy being available.