The U.S. did not sign the Hague Accords of 1899 that mainly delt with the humain treatment of prisoners of war & civilians caught up in the conflect. It also banned the use of poison gas.
The U.S. did sign the Hague Accords of 1907 that delt with the Rules of Land Warfare, including bullets that caused "needless pain & suffering". This time around poison gas was not included. Mainly because Germany was planning on using it in WWI, and didn't want us shooting back with dum-dum bullets!
Anyway, to answer the OP's question.
FMJ is very cheap to produce, feeds well in anything, shrugs off dirt & mud, penetrates heavy clothing & web gear, hard targets like vehicle sheet metal & helmets, and soft cover like earth berms better then expanding bullets.
Dollars to donuts we would still continue to use it, even if we were allowed to use JHP pistol ammo under the international rules of warfare.
Do not confuse the needs of the military with those of cops & civilians.
The JHP's one-shot stops without over-penetration are not even a consideration for military ammo planning & testing.
rc