Why not? The wall wart outputs are isolated from the wall voltage. Voltages are relative, so the -V of one can be the +V of the other. Nothing dodgy about it.
Though whether it’s good or not is another question. Wall wart DC supplies often have poor to no regulation; the actual voltage supplied may depend on how much current you’re drawing. I wouldn’t use this for a synth, but for some simple projects it might be fine.
There are numerous topics here about power supplies, no need to create another one. Right at the top of the FAQ category:
Also, the original post prompted this but it belongs in its own topic: