You can look at this the other way as well. The scanning beam only causes a low exposure at a point because it is scanning very quickly. The peak radiation is quite high. We know the dangers of cumulative dose because that is most useful in the case of low level exposure over a long time. Could it be possible that the risks are different for brief intense exposure?
It is still rather contraversial but radiation hormesis is interesting to read about. Populations have been exposed to relatively high radiation levels from things like the earth (Iran and some places in the US among others), contaminated steel (Taiwan) and various medical procedures. These groups have not fitted into the linear no threshold model very well at all, and there is an argument that low doses may actually be a health advantage.
http://en.m.wikipedia.org/wiki/Radiation_hormesishttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC2664640/
Edit: keep the body scanners and bomb scanner away from me please.
