the reason is that the Gigabit bit rate is too fast for the Arduino processor, so the ethernet shields for the Arduino only go up at 100 mbits ... So as the Cisco switch does not see Gbits, it does not communicate with the POE of the card ...
Well, that's completely wrong...
PoE is fiddled with entirely separately from data communications, and the cisco-500 supports speeds down to 10baseT (MOST switches invisibly handle speed conversion on the data side of things.)
Since this is a feak'n Cisco you're talking about, it has about a billion commands to modify, show, and debug PoE behavior, so a bit of debugging from the switch side might be useful (this may be difficult if the switch is controlled by the IT department and not you 
) For example, there is a "power inline legacy support disable" command...