The keys are matte because of them having some texture. The texture is on purpose, to increase friction towards your fingertips and therefore key feel. People like Topre's because theirs is more pronounced.
I have a keyboard with some very shiny ABS keys. They are shiny because the keyboard is heavily worn, having been used every day for six-seven years. :-P
You can test whether a key is ABS or PBT with some acetone and a q-tip. Dip the q-tip in acetone and smear it on the inside of the key cap where you can not see. If there is a mark, then it is ABS. If the acetone just evaporates without leaving a mark, then it is PBT (or POM).