Well, technically, they aren’t always limited to FMJ. Although they are forbidden "to employ arms, projectiles, or material calculated to cause unnecessary suffering."
Generally, this limits them to FMJ, but in some cases, such as with the MK262 5.56 OTM (Open Tip Match), a HP is used. It is extremely effective within fragmenting ranges (even more so than M193 and M855). However, because it's "open tip" was created primarily for accuracy purposes first and foremost, they get around the "rule".
Contrary to popular belief, this is according to the Hague Convention, not the Geneva Convention.
The US can and does (not in general though) use JHP's, or "arms, projectiles, or material calculated to cause unnecessary suffering."
About 20 years ago, a US judge decided that it was perfectly legal to carry JHP's for special purposes.
"...expanding point ammunition is legally permissible in counter terrorist operations not involving the engagement of the armed forces of another state."