What VW did in the states was very blatant defeat device and the surprise is they got away with it for so long. They used the steering input sensor to disable the SCR system (which converts NOx in the exhaust into harmless gases). When on a rolling road during an emissions test, steering wheel doesn't move, so SCR is enabled, when on the road, the steering wheel is being used intermittently, SCR is disabled. The main reason for disabling SCR would be so the AdBlue tank only needed to be topped up at service, not by the driver more frequently which means that some related OBD rules could be avoided. Bosch who make the ECU and software specifically told VW that the steering input feature was only for development, not for production (during development it's normal to disable functions for various reasons). There is no conceivable legitimate reason to use steering input to disable SCR in production release software. In Europe the diesel emissions limits are easier to pass so there's less reason to try and cheat - but reports are they have cheated in Europe too. Most OEMs are pretty good on the corporate responsibility thing and don't bend the rules much but there are grey areas where the letter rather than the spirit of the law is worked to. However, the VW thing is utterly blatant and completely illegal.