There is no connection between the line voltage and the output of the power supply. This is immediately converted to 20kz or so AC and pumped through a toroid transformer that produces the various DC voltages.
These power supples are designed for a global market and I am sure there are countries that do not use grounded line voltage.
Ethernet rejects 60hz hum anyway. What they do have problems with is large ground shift transients from baseplate ground in one machine to another. (tied to the the EGC)
It tends to fry the card and might even start blowing up other stuff hanging on the bus..
The NEC doesn't make you ground one leg but it doesn't prohibit it either.
