How does the microwave turntable work?

Because their surface has been compromised. For example, stoneware is microwave safe, but once it has a chip or crack in the glaze, it begins to absorb rather than deflect the heat.

If you are not sure if something is microwave safe, place a glass of water in the microwave next to the plate, without touching. Turn the microwave on high for one minute. The water should be warm and the plate should be cool.