Then you'd imagine incorrectly – see above, there's an entire industry around accepting payments for goods and services. The economics are often terrible and the UX sucks for everyone. Square is disruptive because it's better for everyone – the merchant and the customer. The only one getting screwed for a change is the payment processor. Hence, disruption.
How is the processor "getting screwed" here? Again, for a great many businesses Square is the (significantly) more expensive option.
I get that they're killing it at craft fairs, but I'm the with the thread-parent...I don't see what the "disruption" is here.
*edit: Re-reading this I realize that it seems like I'm implying that Square is a bad product... which isn't what I'm saying. I think they're going to make insane amounts of money (I'd invest in them given the chance). They can be huge without being truly 'disruptive', however.
I showed the Square reader to my hippie friend who sells hand-made jewelry in an outdoor booth at a beach resort. He thought it looked pretty cool, and was seriously considering buying an iPhone and service plan just to use it. At his usual transaction size and volume, traditional credit-card payment systems weren't that great a prospect. That's what a disruptive technology does.
About time, too.