The answer is simple, according to Ross Douthat:
Imagine, for a moment, that George W. Bush had been president when the Transportation Security Administration decided to let Thanksgiving travelers choose between exposing their nether regions to a body scanner or enduring a private security massage. Democrats would have been outraged at yet another Bush-era assault on civil liberties. Liberal pundits would have outdone one another comparing the T.S.A. to this or that police state. ("In an outrage worthy of Enver Hoxha's Albania ...") And Republicans would have leaped to the Bush administration's defense, while accusing liberals of going soft on terrorism.
But Barack Obama is our president instead, so the body-scanner debate played out rather differently. True, some conservatives invoked 9/11 to defend the T.S.A., and some liberals denounced the measures as an affront to American liberties. Such ideological consistency, though, was the exception; mostly, the Bush-era script was read in reverse. It was the populist right that raged against body scans, and the Republican Party that moved briskly to exploit the furor. It was a Democratic administration that labored to justify the intrusive procedures, and the liberal commentariat that leaped to their defense.