Well, if that is the standard, then it relies on circular reasoning to support the hypothesis.
I have no idea what you mean.
Are you saying that one can never study the impact of a glass of wine a day on people that drink once a week?
Or we can never find out what amount of drinking water supports a heatly life style because people have to drink some water to subsist and therefore you can never find a control group?
I'll try to explain this one more time. No circles of reasoning here. Usual background radiation results in normal life. So-called "Low level" radiation (which is higher than background) results in lower cancer rates than "normal life". High levels of radiation result in higher cancer rates than "normal life".