Square readers are tangible personal property, not real property; and they are tangible personal property o the customer once they are sent out, not of SquareUp. So, they have zero relevance to a rule that would make real property owned by SquareUp a nexus in the state, as they are neither real property nor owned by SquareUp.
Good point, that was a poor example I chose. This tax will hit service-delivered-as-an-appliance plays out there, if I am reading it right. Unless they give away the appliance server.