Some weapons are intrinsically forbidden because of their effects on individuals: soft-point bullets for instance. These are as discriminate as you want them to be, but are nonetheless prohibited in conflicts. Thus it's not just indiscriminate weapons that are banned by international agreement!
When asked why tear gas is banned, the answer I gave was "the weapon is indiscriminate and chemical" not "all banned weapons are indiscriminate and chemical".
For example lasers used to blind soldiers are generally banned as well and as you point out, lasers are basically arbitrarily discriminate.
