[mseebach]

It's the word "unreasonable".

Regular x-rays and metal detectors can be shown to have stopped a very large proportion of items immediately threatening to passenger safety, and are thus to be considered reasonable. Backscatter have no such advantage (in fact, it seems unlikely they could even have stopped 9/11) and are therefore considered unreasonable.

YMMV, but I don't consider myself "played" for subjecting myself to the less intrusive searches. I do feel like an idiot having to pull out my laptop and toothpaste and having to throw out any drinks.

[astine]

"Regular x-rays and metal detectors can be shown to have stopped a very large proportion of items immediately threatening to passenger safety"

By that logic however, x-rays machines and metal detectors should never have been permitted in the first place because when they were first introduced they had no track record of utility either. You're right that it's the definition of the word "unreasonable" that's the sticky issue, but I don't think that it's as clear cut as you say it is..