How is blinding someone with a laser worse than killing or maiming them with a bullet?
The assorted 'laws of war' are heavily leavened by what their framers suspect that they can actually get at least some people to agree to; but the overall theoretical foundation always seems to be an attempt to steer weapons in the direction of "Kills outright, or leaves a wound that, if treated, will heal with comparatively limited permanent damage."
It's not an easy standard to maintain(both in terms of convenience, mass-maiming is a hell of a shock to morale and logistics, and engineering, something that will kill if it hits you as designed will likely cause serious tissue damage and/or amputation if it scores a sub-par hit); but it's not really a terribly strange shared desire, from the perspective of the warring European powers of the 20th century that wrote most of them.
"Experience has proved that some people indeed know everything." -- Russell Baker