thats called using cheap plastic for the lens. its usually not visible from most angles, you have to shine the light on it the right way. most stock plastic lens dont have any rainbows, its usually the cheaper aftermarket lights or if they used a poor quality plastic formula at the factory.
i don't think it is cheap plastic, i have seen a few lights like that at car shows... i saw a green civic that look nice with them. i also don't think it's only on new lights, it's the way they come. someone has got to know where to get them. correct me it i'm wrong but the ones he is talking about are VERY rainbow and you can see them at all angles.