In that case, losing the ability to use those weapons was deemed to be outweighed by the consequences of having to deal with them being used on you. Also, since they could be potentially huge and unpredictable force multipliers, it made war all the more predictable, which is good from their perspective.

Long story short, nearly all rules of war exist because they are beneficial to abilities of the parties to conduct war. The rules do no exist to make killing fair. They exist so that parties may conduct war in a way they agree with.

That actually makes perfect sense. Although I find it rather disturbing.

http://en.wikipedia.org/wiki/Expanding_bullet#Law

and note that domestically governments do not have to obey even the most basic "rules of war" when they set police against their own people

Secondly the other parts of the convention do not talk about war as a set of specific circumstances (e.g requiring declaration of war) but just hostilities regardless on how they originated.

So by skipping the declaration of war one simply breaks one additional part of the convention, the one mandating the declaration.

Snipers are force multipliers ~ http://en.wikipedia.org/wiki/Force_multiplication

Also a better rifel is less prone to jamming (this is why SF's prefer the piston version of the M4)