USB 3.0 Power Limitations
There are two NON-CONFLICTING requirements in USB specifications.

First, a USB DEVICE must not draw more than 100/150 mA from USB port before it gets enumerated and finally configured. Then, if its power requirements fit into host budget (and the host knows what it is), the device can use more. Otherwise it won't be configured and used by system. 

Second, a USB HOST ports must be able to supply much more current. From Section 7.2.1, "Systems that obtain operating power externally, either AC or DC, must supply at least five unit loads to each port.".

So, if your USB HOST has a AC-DC adapter or is plugged into AC outlet, or has a massive battery, each USB 3.0 port must be capable to feed at least 900 mA.

If it is a skinny smartphone, it depends on how the manufacturer feels about battery life of their product.