It turns out that -48 V is less corrosive. that's why it's used.

the voltage is "relative". Lots of wall warts might have a coaxial power connector. Some may have center positive and others the outside positive.
How you hook it up depends on the circuit.

A -48 supply may indeed have the positive side grounded, but it may not.

A +-12 V supply has a common point and when you measure to that common point you get the +12 and -12 voltage. But that common point may not yet been grounded.

An isolated 48 V supply can generally be connected either way.