To start with the spokes are not stainless steel they are simply galvanized.

Old (Japan built XT600's) used to have proper stainless steel spokes.

The corrosion on the spokes is the bi-metal in the brake pads coming off (very hot, very hot, ouch ouch) as a fine mist that then burns onto the spoke coating. Galvanic effect occurs between the bi metal brake dust and the spoke coating removing the thin coat leaving the metal spokes to rust.

Yamaha had to save money somewhere.