It is a mistaken belief that the Geneva Convention applies to ammunition; it does not. The one that does is the First Hague Convention of 1899, to which the U.S. is not a signatory. However, there is a provision in the Second Hague Convention of 1907 against ammunition designed to cause cruel or extraordinary wounding, and we are signatories to that convention.
It is arguable that expanding ammunition is not intended to cause any sort of wounding at all, but a quick and humane kill. That is certainly the argument for its employment in hunting. Were I to decide, we'd be using it at least in handguns, which do not (usually) develop the muzzle velocity necessary for tissue supercavitation that does occur in rifle ammunition and is the reason that rifle fire tends to be more lethal than pistol fire.
OTOH, FMJ has the advantage of greater penetration of armor. .357 SIG in FMJ would make an excellent combat round.