Except that NN does not prohibit it for traffic engineering purposes. And I'm here, just doing traffic engineering:
if (you-pay-me-for-PNIs) { set-local-preference really-high } else { set-local-preference default }
Oh look, your traffic is going to the non-congested path which happens to be the path to my PNI locations, directly into your network. Jack on the other hand, Jack gets to use the standard transit. "Hey Jack, you know, we will drag all your traffic to SJC, even if you are on the East Coast and hand it off to TATA because TATA gives us the lowest transit price. Man, TATA seem to be congested from 5pm Eastern to about 2am Eastern. Oh well, maybe you guys need to yell at TATA. Oh, do you want to buy a dozen of 10Gs to me all over the country? That would avoid those dolts at TATA."