It should be that all devices have isolation. Ethernet is capacitively coupled which keeps you from having to coordinate your power/ground between devices. Po

It should be that all devices have isolation. Ethernet is capacitively coupled which keeps you from having to coordinate your power/ground between devices. PoE is the recognition that if the DC levels don't matter because of that required isolation, we might as well not isolate in a power stage before the magnetics and deliver some DC power.