Data lines are not expected to source or sink much current. They are designed for signalling not for powering light bulbs.
On low and full speed devices, a differential ‘1’ is transmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5K ohm resistor pulled to 3.6V. A differential ‘0’ on the other hand is a D- greater than 2.8V and a D+ less than 0.3V with the same appropriate pull down/up resistors.
(from some random web page)
A typical small indicator LED is probably designed to draw 20-30 mA. A LED from a torch might be designed to draw 1A or more. Your USB chips certainly won't source or sink high currents on D+.