why might it be less dangerous to wear no sunglasses than to wear sunglasses that do not block UV light

As everyone knows, when it's really bright out you squint and your pupils constrict to tiny dots - as small as half a millimeter in diameter - to limit the amount of light getting in. When you put on sunglasses, the effect is the same as being in darkened room, the pupil dilates to let in more light.

Passage from online:

"Glasses that don't block UV rays may offer some relief from visible light and reduce your need to squint, but the additional exposure to UVA and UVB can be harmful. "Let's suppose you put on a very dark pair of sunglasses," Bizer says, "Your pupil opens up, it dramatically changes in size from half a millimeter to 5 or 6 millimeters. It's an enormous increase, and now you're allowing much more [harmful light] into your eyes than if you didn't wear the cheap sunglasses at all.""