because the rules say so.
I understand why you are making the assumption, it makes sense, but im going explicitly by the rules here, and they simply say that the weapon needs to have the injection property, not that they need to have that property because its being injected instead of the normal ammo.
the whole description of biohacks implies that the biohack is being applied in addition to the attacks normal effects, it does not say anywhere that it replaces the effects, or that it is replacing the ammo you are using, it even says that the biohack is expended in addition to the ammo itself, implying that the biohack is applied in addition to the burny acid effect. (the acid being the ammo itself).
note that the fluff discription of the disintigrator weapons (which all the causto- weapons are, and only the cuasto- weapons and injection glove are) says that they dont actually inject acid, and generate an energy field that breaks down the bonds between atoms, creating an effect similar to an acid. how does this energy field then allow you to inject them with poison instead of the acid like effect? no idea, the causto- weapons are a bit 'wierd science' :D but then so are biohackers *shrug*
its also worth noting that the caustolance does less damage than equivalent weapons, the tier 1 laser rifle does 1d8 damage and at double the range, for example.
I believe this is to balance them with biohackers using biohacks in mind.
