elcentral :
its like some people already stated previously. the device will try to take the ampere it needs and it will not be able to go beyond it. if you know ohms law you will understand that if you got 12V out usb and your electronics for example got 24ohm resistance U=I*R we got U=12V out on usb, we got "R=24Ohm example of a device u plug in to the usb" and you like to know how much I=ampere your unit will try to use. so now I=U/R so in this example we get 12V/24Ohm=0,5A or 500mA so in this case this cable would do nothing. the device will not use more A unless you change the Device resistance or the volt on the usb. but if the device would be at 12Ohm we would get 1A and could possible get more Watt in to the device as it will try to take 1A on the 12 volt usb. but ceep in mind 500mA is a rated Ampere not a law your mother board might let you use lots more. for example almost all 4G sticks use 800-1,000mA of your usb and they work fine so it might work.
sorry for boring you all : ) but i to got stuck in the Ampere to usb questions once.
I don't think the actual concern is the the physics itself. The bigger concern is something non-standard potentially not working on all USB hosts. A cable like this is essentially removing the "universal" from the USB.
Sure some manufacturers will design power delivery on the board to support 1A on the USB 5v rail. However, that's not part of the USB spec and it's very possible that many manufacturers won't be that cautious. In fact, I would assume that most Manufacturers never thought of it and never really designed with that constraint, if the board happens to give 1A off USB 5V, it was an accident...
There are specially designed USB ports that DO support 1Amp, and manufacturers usually denote them with a special color (Red I think?). Those are the exception and not the rule.
If you try pulling too much current, the voltage starts dropping on the USB 5V rail. This may cause undefined behavior in other USB devices (and other things that uses the USB 5V rail). This can compromise system stability.
Some boards have over-current protection and shut the system down... Others will just let 5V droop until stability is compromised and the computer crashes. If traces are done really poorly, the extra heat from 1A could burn out traces on the board. (remember, USB is found on all sorts of "computing" devices, including stuff that costs a few bucks to make. As long as the device supports USB protocols, it can give the device 500ma)