The actual programmer & designer of the software is the supplier of the engine control units (ECUs), in most cases Bosch. The software that they supply actually already contains these cheating routines. These routines are delivered as a part of the software packages, and can be turned on/off with some flags.
It is up to the auto company how to set these flags.
Source: Close relatives working in the car industry.
The emissions standards don't apply on a world-wide basis. Maybe they should. But disabling them for markets which don't require them, and gaining fuel efficiency, is at least not fraudulent behavior.
It might have safety implications (e.g. deactivating airbags so they don't fire because of implausible-on-the-road vehicle states), or even enable certain tests in the first place (perhaps by disabling traction control systems confused by vastly differing speeds per axle).
Using them in order to cheat, however, is clearly not nice at all.
Speaking as a German, and as someone who has worked in the automotive industry, I'd really like for manufacturers to be punished hard on this.
The amount of complacency and disdain for regulations on the part of the manufacturers is disappointing, and the collusion by the German government distasteful.
This very ambiguity makes it easy for both sides to just shift the blame to the other side in case this whole collusion gets discovered – which you can now nicely see in the wild. In the best case scenario for both companies, both just get away with it this way.
https://en.m.wikipedia.org/wiki/Plausible_deniability
Obviously, no parts supplier would integrate these functions in the first place if nobody asked for it...
It might even be a good thing to remove dead code -- what isn't there can't possibly cause trouble.
But it complicates product management: instead of a single binary and a bunch of feature flags, you now have a bunch of binary versions; and the number of permutations can get quite large quite quickly.