It's true that processing is low margin, but (1) you make a ton of money on a low margin as long as you have a lot of volume. Just ask Wal Mart, Amazon, Big Oil, etc. (2) Not quite the same, because verifone is not a processor. Compare to companies like WorldPay and TSys.
I'm not sure what proportion of square's revenue comes from processing vs services, but if I had to guess, I'd say most of the services are loss leaders for the processing. This is especially true with their rates so far above market. 2.75% is a fat chunk of change if they're processing 10 or 12 figures. (Working primarily with smaller clients, you get to charge that much, but it makes them less competitive with larger merchants who process more volume.)