lolThere is no transformer in a DC-DC appliance like that. No voltage regulator, either. Just a simple resistor to turn 12 V into 5 V and a 5 V LED.
Edit: Actually, there sometimes can be circuitry that does regulate it to 5 V, but it's generally found in more complex circuits. Either way, that draws less current than the 5 V light itself, but sure, to be fair, the sum of the current draw was responsible, as the light was on the whole time.

I measured the total current draw of the 12v to 5v USB adapter with my battery at 12.8v and nothing plugged into the USB: 48ma
I cut the lead to the LED so it didn't light up anymore, total current draw: 36ma
Therefor LED current draw: 12ma
I corrected the problem by putting the 12v to 5v USB adapter on a switch. When the switch was "off" the USB port would only be powered when the bike was running (license plate light lead), with the switch "on" it would be powered 24/7 (for charging my phone when the bike was off).
Other 12v to 5v USB adapters might more efficient.
-Jamie M.