^How?
Harry didn't make the complaint. The mayor of Gila Bend, AZ was "quoted" in the Daily Mail as warning against Prince Harry and the other soldiers. Harry wrote to the mayor, thanking him for his city's hospitality, and informed him of the quote in the DM, and gave him the information for the PPC if the mayor wanted to file a complaint.
Had the mayor actually made these comments he wouldn't have a leg to stand on,but if he didn't, then he has a case, and apparently he did.
I wonder, if it had just been Harry that the article attacked if Harry would have just let it go, but since it included remarks about his fellow soliders he wanted to straighten everything out?