The issue with NOX isn't that it exists, it's the concentrations it exists in.
Even the most NOXious diesels are fine outside of urban areas and smogtraps, but a large chunk of California has smogtrapping areas (The central valley and LAX basin), so the laws nationally are set for those (NOX is why lean burn gasoline engines were banned. Instead of regulating NOX emissions and allowing makers to comply any way they wanted, US carmakers lobbied the government to mandate stociometric air-fuel ratios which allow "easy" 3-way catalysts to be used - at cost of fuel economy.
EU rules haven't said much about NOX until recently and now they allow makers to solve the issues however they want, as long as they comply.
Another way of tackling the issue using the technology in the dieselgate engines would be to add a NOX sensor in the intake, switching to low-NOX mode when atmospheric levels increase above a threshold and economy+power mode the rest of the time. The problem is that rules and regulations are based on the assumption that emissions control systems are fixed or extremely slow-acting things and they haven't been for a long time.
Urban NOX levels are the main reason why condensing boilers were pushed so hard by councils. There have been NOX limits on boilers since ~2003, but condensing units emit even less (almost zero) because the oxides are absorbed into the water and go down the drain instead. This is important because around half of London's NOX is sourced from old boiler installations that aren't (currently) covered by emissions legislation - they were grandfathered for 20 years past the introduction of the new law. Those NOX bombs tend to be 1970s-early 80s non-sealed units that also put out shitloads of carbon monoxide.