I'm curious as to what these other activities are....and why taking one risk doing these "other activities" justifies taking another risk by riding a bicycle without a helmet.

I've heard anti-helmet folks carry this argument to the extreme and say that we should wear helmets to do everything, or wear full body protection while walking to prevent any sort of injury whatsoever. And to that I say, if you want to carry personal protection to those extremes, go ahead. But that doesn't justify wearing no protective equipment (cycling helmet) at all just because it won't protect against anything and everything.

You wear a seatbelt while driving a car don't you? Driving, in and of itself is certainly not dangerous when done correctly. But, as you point out, it becomes dangerous when other people voluntarily make it more dangerous via their own actions. Unless you cycle in a vacuum, you are likely to be exposed to these very same people whose actions you must guard against by wearing personal protective equipment.

And even if you do ride in a vacuum, no one is so perfect that they are above the slight possibility of hitting a crack or pothole in the road and taking a header...which, in my opinion, is the accident where a cycling helmet can be the most useful (head first over the bars).

BTW...I'm pro helmet, but certainly not pro helmet laws. To each his own.