Function of this voltage regulator circuit?

Working on a data over power bus system, I’ve seen the following circuit in the power decoupling circuitry. If I am right, it gives a regulated output voltage by means of the Zener diode.

^{simulate this circuit – Schematic created using CircuitLab}

• V_BUS = 15 V
• V_ZD1 = 8.2 V

And:

• V+ – V_out = GND – V-

• V_out ~ V_Z – (V_beQ1 – V_beQ2) ~ 7 V; Respecting to GND, assuming that V_beQ1 = V_beQ2 = 0.6 V

I have seen other circuits than outputs a regulated voltage using a Zener diode and a mid power transistor (Transistor Series Regulator) having the same negative reference or GND.

My question is: why set a difference between GND and (over) V-? Does it have to deal with the data AMI encoding?

If I am wrong or need more information, please let me know.

Thank you.