That's a good read. It explains precisely why you're not always saving money by taking it to someone else.
Between the lines ... if you have a failure in your vehicle that is the direct result of the non-genuine substituted component (eg an oil filter), then the repair bill is YOURS and you have to claim it back from the supplier, installer and/or manufacturer of the component, whereas if the dealer serviced the vehicle, THEY are responsible for the repair and you don't have to worry about it.
It's absolutely true that Nissan should not be refusing warranty work if the vehicle is serviced by someone else AND the fault isn't to do with that service or the components used. It may be difficult to prove, and to avoid the problem entirely I'll continue to have my vehicle serviced by my dealer (whom I have had no problems with at all).