[throwaway2048]

NOx are industrially useful and profitable to scrub out at large scale (and relatively easy too, just pass exhaust gasses through water) because it generates nitric acid, which is an extremely useful and relatively expensive chemical that is the basis for vast amounts of industrial processes. So the use of ammonia in power plants is relatively unproblematic, in fact I would not be surprised if tuning ammonia based power plants to produce the most amount of NOx possible was the most economic choice.

As a bonus, nitric acid can be directly used to create fertilizers, which is one of the central usages of the Haber–Bosch process.