This isn't a casual mistaken. Taking from the newly implemented border exit control that we've gotten. (If you think we don't have one, re-read that) :

The GAO found that the ability for people to understand that Americans are not required to go through the biometric exit was non-existant and the experience of opting out was very poor.

What this means is signage was not posted that indicating for Americans this is an optional process and people forming the "requirement" were not educated that it is optional for citizens.

Yet, the experience is that people forcefully push people into to posing for the camera with markings on the floor, the lack of opting in/consenting to it, and prevent people from being aware of what's going on. (Yes: You can opt out .. walk up to the board area with your passport open to your photo page)