There is an inherent comfort level with ATMs that is absent with non-FF travelers. I think that they are afraid of hitting the wrong button and canceling a ticket, changing a flight/seat, or something else out of their control. They have the comfort level only with a person, as they have become accustomed in the past.

I have also had to step around these folks, even in the FC/GS lines - of course in those lines the idea is really that the GA/TA wants the chance to talk to us, not vice versa!