Ok, I just spent some time reading the Geneva Convention and really couldn't find anything about it. I got this off from wikipedia though.

History

Full metal jacket ammunition is acceptable for military use by the countries that signed the Hague Convention of 1899, which prohibits the use of hollow point or expanding bullets in war between the countries which signed that agreement. It is often incorrectly stated that the prohibition is part of the Geneva Conventions, and that full metal jacket bullets are specifically required. Although the actual language is rather vague, full metal jacket bullets remain the best option available to stay within the letter and the spirit of the Hague convention. The bullet itself is still highly lethal when fired at close range.

Advantages [ul][*]Because the bullet does not expand as do hunting bullets, FMJ bullets are more effective at piercing armor. Similarly, they better penetrate moderate cover to reach an enemy behind moderate cover, e.g. sheet metal portions of a vehicle.[*]They are more durable and withstand rough handling on the battlefield.[*]Their rounded tips permit proper transit up the feed ramp of a semi-automatic pistol, whereas hollow-point bullets can present greater occasions for jamming; (this risk is reduced by using high-quality firearms and ammunition, and by conscientious gunsmithing).[/ul]So I'm not really sure that they are against international law, just pretty much accepted as a standard is all. Plus it works well for the intended purpose.

Paul