Andrew Gabriel wrote:

I've never yet seen a USB port which actually polices
the current draw against what's been negotiated.

I've encountered laptops that do, but not recently. I suspect that the
abundance of cheap "USB"-powered devices that don't ask nicely for power
means that all modern hosts will supply the full 500mA regardless,
otherwise they (rather than the out-of-spec device) are perceived as not
working properly.

Pete