Originally Posted by WhyFi

It's not really a matter of improper design. You can polarize two ways: linear or circular. Linear is more common and less expensive, but you'll have issues if both filters aren't aligned in the same direction and, even if they are, visibility can still suffer as you move about. This isn't really an issue with CPLs, but they're definitely less common; I'm not even sure how many sunglass manufacturers make them.