This conundrum clearly falls thus:
1) Bono has Glaucoma and wears sunglasses to protect himself from natural and artificial light during, for example, concerts
2) Bono is using Glaucoma as an excuse to wear sunglasses. Please see 2a), 2b) and 2c)
2a) Bono thinks it makes him look cool
2b) Bono is insecure hence the sunglasses
2c) A bit of both 2a) and 2b)
3) Bono has a different, undisclosed condition, for wearing sunglasses