So let's take your 0 V and -12 V example. From the positive regulator's point of view, you don't have 0 V and - 12 V. Its GND, that is to say it's local 0 V, is whatever is connected to its GND pin. From the point of view of everything that's attached after the regulator, regulator's GND is its own GND and regulator's Vcc is its own Vcc.
What you've done here is basically replacing the rest of the circuit with Thévenin's generator. The load doesn't care what you have on the other side because it will simply see the positive voltage.
Let's take a look at this simplified circuit:
Here we have the -12 V connected to the GND pin of the regulator, the outside GND connected to Vin of the regulator and we have a load connected to the regulator. The voltage on the load in this case will be +7 V, because the load's ground is same as regulator's ground. We've basically created a new virtual ground from which we can count voltage, however the main point is: There's no big difference from load's point of view between this and just calling the GND +12 V and the -12 V GND. You already need to have the negative voltage from somewhere and negative regulators are usually there to solve the somewhere problem. Also while the voltage at the load will be -5 V from the point of view of the main ground, it will be +7 V from the point of view of the load.
Here's another circuit which uses the same reasoning as the previous one:
The main difference here is that the load is actually seeing the voltage of +5 V with respect to its own ground. That voltage is -7 V with respect to the main ground of the circuit, but the load itself won't actually see that, since the load's ground will be tired together with the regulator's ground.
Now let's take a look at a 7905 circuit:
You have same ground between the load side of the regulator and the input side of the regulator! This is important! In almost all situations where the term
negative regulator is important it is so because you already have the positive regulator somewhere. Devices which need to use both positive and negative voltage (such as operational amplifiers in some configurations) will need to have the positive voltage with respect to the common ground and negative voltage with respect to the common ground. While with your example, you will get -5 V with respect to the main ground, you'll already need to make the negative voltage somehow.