Earlier quoted context omitted.
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?
http://en.wikipedia.org/wiki/Linear_no-threshold_model If it holds, it's 'good' for the scanners because their high power shouldn't be penalized; bad for them because any dosage increases risk.
Edit: keep the body scanners and bomb scanner away from me please.