That may be a Raspberry Pi problem. The Pi's non-power USB ports are "On the go" ports, and potentially bi-directional. But they don't have the proper power circuitry and protection on the power side. The Pi's power USB port doesn't do power negotiation, so if connected to a power source that demands negotiation, it may only get the default per-negotiation 100mA, which isn't enough.
(I've been designing a board that gets power from USB, and had to read up on all this. The USB power system is complex but well designed, and if all the devices comply with the standards, immune to user mis-plugging. The problem is that doing it right usually requires an extra IC at each USB port just to manage power.[1] A lot of low-end devices don't do all this right.)
(I've been designing a board that gets power from USB, and had to read up on all this. The USB power system is complex but well designed, and if all the devices comply with the standards, immune to user mis-plugging. The problem is that doing it right usually requires an extra IC at each USB port just to manage power.[1] A lot of low-end devices don't do all this right.)
[1] http://www.linear.com/product/LTC4085