Bono's head disappeared up his own posteria years ago and, mainly because his belief that he poos chocolate is actually true, it's very dark up there. The glasses you refer to are not really shades; they simply help him see in that darkness. And, like Edith Piaf, he regrets not going to Specsavers.
Bono himself has said that "It's part vanity, it's part privacy and part sensitivity�. His eyes are very sensitive to light and his right eyeball swells up if exposed to bright light for too long.