the Geneva Convention requires full metal jacket bullets in combatant arms,
And the Hague Convention does NOT "require" the use of full metal jacket. They attempted to outlaw the use of expanding point bullets that created larger wounds. In the late 1800's the state of the medical arts was still very poor, amputation still a significant treatment for major gunshot wounds. The idea was to reduce that.
The military still has a primary requirement to use a penetrating design in bullets, cover being still used as protection against gunfire. Bullets need to go thru walls, sandbags, and vehicles to reach the occupants. Our military kept the use of FMJ to meet that need, NOT to conform to an International Treaty. We didn't sign up, either.
In the '80s JAG came out with a legal opinion the use of open tip bullets designed for better aerodynamics was legal, which mostly affected long range snipers using Sierra match bullets. You don't see them issued willy nilly to soldiers because they have poor penetration on hard targets. Hollow points dump the energy without passing thru. Not what someone shooting back at a mud hut needs, but it does reduce collateral injuries, which doesn't affect the sniper much.
A civilian concept of what makes good ammo isn't what the military has in mind. The military doesn't need or want it, combat is that different.
There is now a school of thought the use of hollow points by our LEO's is being MORE humane - it takes less shots to stop someone, and they suffer less from one hollow point gunshot that 5 - 10 FMJ rounds perforating them. That increases the rate of survival, and not all of them were guilty, which is another advantage for the use.
Vet 22 years, this seems to pop up more now than years ago. The public myth about the Geneva Convention has become much more distorted since the days when 1 in 10 had served in the '60s. Now, it's 1 in 100, and military issues are largely misinformation and rumor.