What is the justification for the difference between "venom" and "poison"

Solution 1:

TLDR: the distinction actually exists in modern-day French, and this may be the source of the restriction of venom and venomous to injected toxic substances.

Walker's pronouncing dictionary from 1828 has the definitions

poison: that which destroys or injures life; venom.
venom: poison.

The 1892 Webster's High School Dictionary has the definitions

poison: Any substance noxious to life or health ; venom ; ruin ; malignity.
venom: Poison; spite; malice.

So it appears that, in the common language at least, there was no difference in meaning during most of the 19th century. The difference certainly exists today in the scientific literature.

The distinction between venomous and poisonous may come from French, which has two adjectives that originate from the same word in Latin: venimeux and vénéneux. Today, the Larousse dictionary says that French uses venimeux for toxic substances that are injected (or maybe absorbed through the skin), and vénéneux for toxic substances that are ingested (or maybe absorbed through the skin).

The TLFI says:

La spécialisation d'empl. (vénéneux à propos du monde végétal et venimeux à propos du règne animal) plus marquée dep. le xviii^e s., n'a cependant jamais été complètement réalisée.

The specialization in usage (vénéneux for plants and venimeux for animals), more marked from the 18^th century on, has never been completely realized.

I don't know how the distinction in French between venimeux and vénéneux came into existence. It certainly seems to predate the distinction between venomous and poisonous in English, which might indicate that we got the distinction from the French.

Solution 2:

As explained by science.org the main difference between venom and poison is the way they are delivered to the victim.

The terms are often used interchangeably, but ‘venom’ and ‘poison’ are not the same thing. True, they’re both a toxic substance that can potentially harm or kill you, but the main difference lies in the way they are delivered to the unfortunate victim.

Poison is a toxin that gets into the body via swallowing, inhaling or absorption through the skin. Poisonous animals tend to be more passive-aggressive—they often won’t actively attack their prey, but release their toxins as a result of being eaten, touched or disturbed. A cane toad, which secretes toxins from glands on each shoulder, is a poisonous animal. It has to be ingested or licked to cause harm. Poison ivy is an example of a poisonous plant—touching it can result in an itchy and sometimes painful rash.

Venom is a specialised type of poison that has evolved for a specific purpose. It is actively injected via a bite or sting. Because venom has a mixture of small and large molecules, it needs a wound to be able to enter the body, and to be effective must find its way into the bloodstream. For this reason, venomous animals are more active in defending themselves. A taipan, which injects venom through syringe-like teeth, is a venomous animal. So are jellyfish, which inject venom into skin using venom-filled harpoon-like structures that shoot out from cells along their tentacles when touched.