USB PC Current Limit
USB 2.0 standard which permitted 100/500 mA was eventually extended with Battery Charging Specification which allows the device to draw up to 1.5A after it sees that the other side (the plug pack) is shorting D+ and D-.

Some PCs may be able to source more than 500mA, but it is not safe for them. And some will allow only 100mA prior to enumeration.