If Riddick's eyes are so sensitive that he is blinded by a flashlight (as seen in Pitch Black), then why the raging hell does he lift his goggles to watch the sunrise on Crematoria, an event that would probably sear his retinas?
Mostly because it's hard to do a subdued Oh, Crap expression with your eyes covered, one would assume. It's either have him take off the goggles so he can emote (resulting in complaints of "Why does he take off the goggles?!"), have him overact to still convey expression without his eyes ("Why is he doing so much Narm?!"), or have him not be able to emote much in those moments ("Why is Vin Diesel Keanu Reeves?!"). Basically this movie has him acting a bit more "human" and less of a relentless combat machine, so Diesel needed to be able to show his eyes more.