So the node must accept power from multiple lines, any of which could (through fault, incompetence, or malicious intent) be anything but a "safe" isolated extra-low voltage.
It needs to do this even if the CO was providing nice 48v telco power over copper to the node. If you assume that fault or malicious intent need to be protected against, you have to worry about 240 volts coming up the line from a subscriber's house. Heck, if you want to be extra careful you need to worry about 7200 volts coming up the line (if a tree falls breaks the overhead power and phone lines, and crosses the exposed copper) though it probably would be rare enough you can just eat the cost and replace the whole thing if this happens. Protecting against 240v AC on a line intended to have 48v DC is easier than protecting against both 240v AC and 7200v DC...
Biting the hand that feeds IT
