His point is that he thinks that many people with the square card reader never read any cards. He's right - unless there's a government subsidy that I don't know about, Square does not make money from every user.
I appreciate the help, but that's not actually my point. My point is that Square does not keep the 2.75%. It is doled out as cost of goods sold in the form of interchange fees back to card networks and issuing banks. No one has any idea how little of that fee that Square keeps but I would guess around 0.2 - 0.5% at most.