Ill turn it back into a discussion.

This whole 'whats immune to what' argument has been going for as long as i can remember.
As soon as Dark types were here and invulnerable to Psychic attacks all the Psychic types turned into Darth Vader and used Psychic powers to throw phyisical objects at Dark types and expecting the ref to count it as say a Psybeam or a Psychic.
As stated by Chris the type immunities and resistances were brought in to make the game balanced.
Does it make sense that Gyarados is a flying type when its never flown and should obviously be a Dragon?
Is it right that Flygon gets to stay a ground type despite being a flyer from its second form up?

There are plenty of grey areas if we try and explain them but none of these situations need to be explained. The rules are there for balance. They were decided and set in stone by the games.
Bottom line is you dont have to understand the rules, only following them is required.