USB 2.0 High-Power Circuit
For USB 2, a device cannot take more than 100mA until it has enumerated with host and it has host permission to draw more. Host keeps track how much devices have already requested and how much is still available for more devices, and based on this it either allows the device to use requested power amount or not. Basically, a device can't draw 200mA without asking.

For USB 2 chargers, the charger is just a power supply and has resistances to indicate to device that how much the device can draw.