My interpretation of you guys answering the question.
"It's poison because it's a plant hurrrrr."
I want a specific answer.
The best answer I can come up with is that in the first Gen Grass Poison was a commun combo, much like Rock Ground for instance, so the guys at GF probably felt it was a little wierd that the Grass starter would be pure Grass.
Also, you're not questioning, let's say, Vileplume's typing. If you don't find it wierd that a flower pokémon is part Poison, then you have no reason to doubt the typing of a toad with a flower growing on his back.
"Gosto de todos, menos do Ramiro, que e repetente" - Bruno Aleixo