I worked at a sunglass hut part time during college. The _vast_ majority of sunglasses I sold were for fashion, not function. People weren't paying $250 for the lens, they are paying $240 for the style and 'real' D&G logo and $10 for the lens. In my time there I would say the only brands you should buy for performance are Maui-Jim (great lens / polarization) or Oakley (impact safety), everything else is junk lens put into fashion frames, including Ray-Ban, though their glass lens were at least passable. EDIT - I mean amongst the brands they carried, I'm sure there are other good lens manufacturers out there.
What I'm mostly saying is that I don't see this as rent seeking, it's just people willing to spend large amounts of money for fashion.