I think it helps them in their own minds. They view the typical checkpoint and observe a small number of people opting out by choosing the frisking & genital grabbing over the Cancer Machines. But, there people are just one of many layers of opt-outers. Let's make a list:

1. People who have opted out by not flying anymore.
2. People who have opted out by taking another form of (as of yet) non-harassed transportation.
3. People who have opted out by flying out of airports without Cancer Machines.
4. People who have opted out by self-selecting a lane a safe lans for various reasons.
5. People who, as a last resort, have failed all of the above steps and are forced to make the "cancer or groping" opt out.

So, opting out is as layered as the TSA's view of security, as Kippie hatched it several years ago. If you add up all of the people executing at least one of the opt out scenarios, it becomes a non-trivial number that the TSA has no interest in calculating.