Originally Posted by
TLHarrell
The majority of LEDs out there are very happy with 3-3.3vDC. Some are rated to be supplied 5v, but may be more difficult to get. 5vDC from a USB port is fine for 1 LED in series with an appropriate dropping resistor. Two in series (2.5vDC) will likely either be super dim or not light at all.
mA rating for power feeds is rated to the weakest link in the chain. To draw a 5A load, you would have to have a USB cable with thicker gauge wiring, which would cost more to produce. Beyond that, all the traces on the board from power supply to the USB ports would have to be thicker and likely shielded somehow to eliminate induced currents to other traces. This all adds (in the manufacturer's mind) needless cost.