You can always opt out. You'll get a pat-down instead and they'll make you wait 5-10 minutes, but I always do that unless I'm running very late. If enough people do, they usually get fed up and open the metal detectors.
Back when they had the backscatter x-ray machines at Sea-Tac, there always seemed to be one lane where the metal-detector was being used instead.
Don't know if this was a coincidence (I don't fly that often), or if they hoped that trouble-makers would chose that lane, and save them from having to do too many pat-downs...
You can research this a bit on FlyerTalk -- the term favored by body-scanner critics for this situation is "SDOO" (self-directed opt-out, when passengers have a choice of screening methods based on which line or checkpoint they choose). SDOO opportunities appear to be getting rarer in the U.S. over time.
TSA is also giving people an incentive to sign up for TSA Pre because (at airports that support it) the Pre line is always metal-detector-only. Right now they are "testing" Pre and randomly directing large numbers of passengers through the Pre line, but the theory is that it will be available only, or mainly, by subscription in the future, which includes some kind of background check. I'm not very comfortable signing up for one kind of government information collection to avoid a different kind, but people who don't like the scanners and don't have that objection might be interested in signing up for Pre.
Don't know if this was a coincidence (I don't fly that often), or if they hoped that trouble-makers would chose that lane, and save them from having to do too many pat-downs...