Because whether or not it is "correct" in some meta-sense, it probably isn't the style used by whatever organization they're doing the editing for. Organizations tend to have style guides for external documents. (And may go off different style guides/rules for different purposes.)
Because double spaces mess up their formatting tools. So they need to purge them always. If they miss one sometimes it means they need to go back, remove the errant double space and start over. Bonus getting a proof back and the customer put the double spaces back in.
