The price I paid for the bowling ball has nothing to do with it.
OK, forget the example of buying it from UPS.
What nn says is that UPS has to charge me the same price for shipping a bowling ball no matter who I bought it from.
They have to charge me the same shipping if I bought it from Dick's and carried it into the UPS store as they would if I bought it from Academy and carried it into the UPS store.
They shouldn't care who I bought it from - they should just ship it at their appropriate rate.
In a micro view, I use a similar approach for a VMware rack of server blades. The bursty network traffic with users editing files and reading e-mail is segregated from the network attached to the NAS with all the file storage. The NAS network is often saturated with file copying and compiler activity. A 3rd network carries privileged administrative traffic to configure and control the server racks. There is no incentive to build that quality of traffic separation with NN in place. Everyone is forced to swim in the toilet together. The jackass streaming the movie doesn't care if you lose your train of thought writing a document with 15 seconds of delay between entering a keystroke and seeing it echo back to the screen.