Historically the idea of war having rules has had a very unfortunate past. In WW1, Germany began unrestricted submarine warfare in 1915 and torpedoed the passenger liner Lusitania (filled with neutral civilians). There's a long list [1] of hospital ships deliberately sunk in WW1.

Then in WW2, aside from even more unrestricted submarine warfare and sinkings of hospital ships [2], we also have the firebombings of Dresden, Tokyo, and every other major axis city; machine-gunning of shipwreck survivors in the water; the atom bombs; the impressment of Koreans into service as "comfort women" for Japanese servicemen; forced labor at both axis & allied prisoner camps; the internment of Japanese-Americans in concentration camps; and of course the Holocaust.

Modern-day, there's the My Lai massacre and Obama's attack on a Doctors Without Borders hospital [3]. Probably more too, but you don't hear about them.

The author cites that war has rules because rules are written down, but rules are written down for Wall Street as well. They're just not enforced. And similarly, the laws of war are only enforced on the losing side, or on scapegoats that the actual decision-makers make available as a token sacrifice. When it comes to actually conducting a war, belligerents usually follow just one rule: win.

[1] https://en.wikipedia.org/wiki/List_of_hospital_ships_sunk_in...

[2] https://en.wikipedia.org/wiki/List_of_hospital_ships_sunk_in...

[3] https://en.wikipedia.org/wiki/Kunduz_hospital_airstrike