I’m unable to quote any laws or regulation here, but take a closer look at it, in the end the cases are different:
License
As I wrote in my answer to your related question here, in case 2) you only get some sort of temporary driving licenses. Whereas in case 1) you will need a full-blown Chinese driving license, international licenses are not accepted.
Car Registration
Again, in case 1) there is nothing special about the car, but in case 2) you are temporarily importing a foreign car and in just about any place in the world there is paperwork etc. associated with that.
So I see it from the point of view that scenario 2) is a fairly special case for the authorities since you do not have a Chinese license and import a car. Instead of saying no they do allow this, but under the special conditions of a needed tour company + guide.
Credit:stackoverflow.com‘