A possible explanation:

Napolitano and Co. are allowing the problem to occur at the border. It not only lets people through, but also substances.... illegal substances.

Under the pretense of security, they are tightening the screws to internal air travelers. But what they may really be looking for is illegal substances, drugs as well as WEI.

One hand lets bad stuff through, the other hand searches people at the checkpoint.

End result is A) conditioning people for warrantless body searches in more areas than just airports, B) get people to push for national ID, C) they enact more draconian measures, etc.

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