Do you want the actual answer or the FlyerTalk answer?

The actual answer is that your dad has a different method of risk assessment than you do. He places more faith in the government and assumes that they would not put him through anything unsafe (most media reports, after all, claim that the scanners are safe). He may not be concerned about long term health risks, and he doesn't see himself as guardian of the constitution. Instead, he is more concerned with immediate efficiency and comfort than you are.

The FlyerTalker answer is that he is "ignorant"and/or an "idiot", as others have said.