They don't. In a system with a Core i7 Gulftown (with six cores), 6gb of ram, a P55 based mobo, a 1,100watt PSU (read: a lot of wasted power due to lack of efficiency), a SSD and a few other bits and bobs, this card peaks off at somewhere around 450watt total system power draw when running furmark (referred to by THG/some other review sites as a 'power virus').
If you cut out all of the rest of the PC, that will still come to somewhere around 250watt on its own, sure, but that's hardly 500watts for just the card...
Average power consumption for benchmarking is around 370watts while more realistic gaming power consumption would likely average out to roughly 250-300watts of total system draw for a single-card system.