Rusty Under the Bottom Bracket
 

You know the screw that holds the cable guide to the bottom bracket? Why is it made from a material that rusts? Can it be rplaced with a metal that doesn't rust?

Sure, get a matching stainless steel or aluminum screw.

Sure, you can replace it with a stainless (corrosion-resistant,actually) screw or even a Nylon one. The screw is under little stress and is only needed to keep the guide from shifting when the wires are loose. Put a little grease on the wires to help them slide smoothly and the screw to help keep it from seizing and the BB (if steel) from rusting. Extra style points for using Tef-Gel.

Why in God's name would the manufacturer not do this?

"Why in God's name would the manufacturer not do this?"
Because some bean counter got a fat bonus for saving a half a penny on the cheaper screw.