I don't know much about the rPi, but the USB spec says the current drawn from a port will remain under some limit, I think it is well below 100 mA when initially powered on. It then has to register with the host as a higher current device before it can draw the higher current. So the current switch in the rPi is likely related to that. It is not clear to me if the switch in the rPi is to limit the current it draws (a poor way to do it as this will cause it to crash) or if this is for the current going out the USB ports where it is part of the spec for a host.
Actually, thinking about the numbers, I think this is the current drawn as a slave since the total current by spec should be no more than 500 mA on any one USB port. So their 600 mA limit is to allow the 500 mA to be reached but not significantly exceeded I suppose. Then if you know you are running from a PSU that can provide more power you can supply more to your downstream USB ports. Just speculating really.