Ball hits base, then rebounds into runner standing on foul ground.
Ball is touched/touches fielder in fair ground, then goes to foul ground and hits runner.
There are several ways it can happen. ASA rules address this. Apparently, USSSA rules do not.
Ok, yes can happen but I have yet to ever see it happen. And as you said the rules do address those situations but as to the OP, Im not seeing how the umpires could be ruling the runner out for being hit by a line drive while in foul territory.