I think what they meant is that "I opt out" is more confrontational and it might trigger a power struggle. "I can't go through the scanner" is more amicable.

In other words, the TSO might give a more thorough pat-down to someone he/she perceives as opting out in order to send a message or cause trouble, and just a cursory pat-down to someone who is perceived to have a legitimate reason in their eyes.