It's not really comparable.

It's standard for whitewater (it's fairly common to be upside down with your head banging on rocks). It's standard (pretty much) for surfing. In other cases, people don't use them (in these cases, it's very unlikely that you'd hit your head on something at any speed). Keep in mind that the speeds are around 3 mph.

Anyway, I'm not advocating for bicycle helmets (that really should be clear). I really don't care whether cyclists use helmets or not. I am just pointing at crappy arguments.

The arguments for helmets might not be very strong. But the arguments against them are not any better (some of them are quite awful indeed).