I can answer your question in more depth if you are still interested.
As Paul pointed out it has to do with electrolytic action. In the early days of telephone the outside plant cables were lead sheathed and referenced to the battery positive terminal (-48 VDC systems). This kept the lead from erroding and leaching into the soil.
On the other hand radio transmission (microwave/sat) equipment uses +24 VDC with the negative terminal referenced to ground.
These two schemes exist today. You are correct neither polarity has to be referenced to ground, but it is still done as to facilitate simple and econnomical overcurrent protection systems. Otherwise you would need to use ground fault protection/detection, and use shunt type breakers on each polarity in primary and secondary circuits for protection. Hope that helps.
