Regarding drag, there's the Shutter Precision dynohub, sold for example as a Supernova Infinity 8. You can mechanically disengage the dynamo part when you don't need it, reducing drag.

Regarding the electrical specifications part, I believe the German StVZO regulations pretty much dictate the rules here. Dynamo powered lights are mandatory for most kinds of bikes in Germany, making it probably the biggest single market for these products. The electrical specs are defined in legislation as well. So, if you are in the business of manufacturing dynamos or dynamo lights, it makes sense to build them StVZO compliant.

--J