Using a shunt zener as a negtaive voltage ref?

If you search Mouser/Digikey for shunt zener voltage reference you mainly get devices for positive voltages.
This may seem like a really dumb question but is there any reason why I should not use any of those positive voltage refs to generate a negative voltag ref? (e.g. I connect them via a current limiting resistor to the negative rail instead of the positive. Anode to -12V, cathode (via resistor) to gnd)

A two-terminal zener/reference can be use to generate a negative voltage by just connecting the cathode (plus connection) to ground and the anode (minus connection) to the minus voltage through a bias resistor. Three-terminal references generally cannot be used that way.

A 3 terminal positive regulator, e.g. LM7805, can be used as a negative regulator if you designate its output as ground, and the negative terminal of the unregulated input as -5V.
This obviously won't work if the unregulated negative terminal is already used as ground.