Flight attendants are "strictly regulated" on your planet? Sounds nice.
The only way I can make sense of your arguments is if you are making two unstated, and incorrect, assumptions. I don't like to put arguments in the mouths of others, but I can really see no other way to reach your conclusion. Feel free to correct me if I'm wrong.
The first assumption is that the background radiation level is uniform across the planet.
It is not. And cancer rates are not positively related to the average background level of a region. In fact, they seem to be inversely related. Here is one link, the first one I found. Feel free to google as many more as you care to look for.
The second assumption is that life that evolved on a planet with background radiation, but without any mechanism to repair radiation damage.
This is also wrong. Cancer happens not when a cell mutates, but when a mutated cell is not rejected by the body, or not quickly enough. Long living animals, like humans, have lots and lots of repair and rejection systems. We call it hormesis when those repair mechanisms are stimulated by subacute doses.
Also, HWE does not appear to be dominant. See here for example.