We design commercial kitchens with lots of stainless steel, white tile, and fluorescent lights. Why do our cooks prefer dark, mat finish tiles?
I would imagine 2 reasons why. First, as a kitchen designer and an ameteur cook myself, I might prefer a darker matte finish for a backspash to reduce the amount of visible "water spots" on the tile. Also, a matte finish might hold up slightly better for professional cleaning purposes, I would imagine.
Second, if I were working all day in a kitchen I might prefer a matte finish to reduce the amount of glare or shine from a glossy tile/stainless backsplash. To cut down on the "industrial" look.
I would imagine that the preference is mainly aesthetics. People do like to have some contrast in their environment and white tiles and stainless steel are both very light colors, thus not contrasting much with one another. The glare from the stainless would be unpleasant and tiring. The darker tiles would reduce this glare by absorbing some of the light that is bouncing around the room. This is particularly true with the mat finish. It is simply because the tiles are absorbing some of the light that might have bounced off the stainless (becoming glare) had you used the white tiles.