Toxic? Well, maybe it's a kinda in-joke with the developers or something. Although, if it was, we'd probably have heard why by now.
Actually, to be honest, it's just one of those useful "all-round" moves that any Pokémon should be able to use. By this, I mean EVERY Pokémon can also learn Protect too (except Regigigas, but that'd be just cheating.) The same goes for Endure etc, so maybe many all-purpose TMs just got a "global" use so the Trainer can just choose how to strategise, and use them. So, there doesn't really have to be a reason for all Pokémon being "poisonous". Unless of course, it was passed down from Mew or something.
Either that, or all Pokémon secrete some kind of defensive toxin, which is most likely. You can't argue that they won't have such glands, because in the Pokémon world, real-world biology doesn't count for anything (e.g. breeding, seeings as every species lays eggs.)