Background levels are around 1 mS/year. So why advocate thresholds more than two orders of magnitude lower than what people normally get in a year? I just don't think science has much to do with your choice of thresholds.
This is a fallacy. The threshold should be set on the estimated benefits of a higher threshold vs the estimated harm from the additional radiation. The background radiation has nothing to with it.
It would be a fallacy if background levels were fixed and unavoidable. They're not. So long as people are allowed to and choose to travel by air, and live in areas with above-average background radiation, it is reasonable to argue that nuclear power should be held to a similar standard.
(Granted that medical imaging is different because you would normally be doing it for a good medical reason.)