I'm not so sure about that. I'd agree that they all have "fancy" (for want of a better word) payment systems, but I don't think there's a reason they have to be the fancy ones made by Square.
But more importantly: the US is (finally) moving to chipped and tap-to-pay transactions. That means that pretty much all merchant hardware is going to need to be replaced over time, so the market the Square (and everyone else) currently has is up for grabs. If I were looking at investing in Square that would make me nervous.