I honestly believe that their thinking on this (upper management) was that, if you came through the first time and alarmed this would give one of the screeners standing around with a wand in their hand not wanding something to do. One of the hardest criticisms that TSA had to overcome was the Thousands Standing Around concept. If everyone was wanding, or doing a bag swab or recycling bins or getting into someone's bag full of dirty , crusty shorts then as least some of the criticism would go away. When the criticism didn't go away then their next solution was to start laying off screeners by the thousands. I know that many of you, CarmelGreg and Spiff particularly will agree with that solution, but what that did was to put our numbers so low as to jeopardise lane security.
The simpler and more logical solution to the probablem would have been to better utilize the personnel that they had at hand. Once again, however thanks to micro-management (at least at my airport) authority was never given to the checkpoint supervisors or even the managers to utilize and schedule their screeners as needed. To this day all screener utilization and scheduling is done up in Ops. Heaven forbid that the checkpoint supervisors and managers should make any decisions on this or any other matter without going through Ops first.