Earlier quoted context omitted.
It's more like paying for the privilege of operating a monopoly on poker tables, with the guarantee that the rake will be kept low, so that the operator is not competing with other entities for the customers' rake. A market maker's competition to collect the spread is with other market makers, just like a casino's main competition to collect the customer's rake would be a different casino.
Read Reg NMS before you opine on this. It's short!
Returning to the earlier poker table analogy, I mentioned that the operator with a local monopoly on poker tables, would be required to keep their rake low to keep their local monopolistic privilege, as an analogy to how market makers also have to keep their spreads in line with the NBBO (due to Reg NMS), in order to keep their privilege of executing orders on an ATS.