The only reason I can think of for the warning is, possibly because the power draw of the drive might be more than the front ports can handle (due to longer leads) and may cause a "brown out" type condition (which will damage a hard drive).
Or, the power output on the front USB ports is less than the rear, having the same effect.
I know that my computer will crash if I hook my video card to the same lead as my boot drive and I've known of people whose hard drives have failed after extended use on the same lead as a power hungry video card.