Originally posted by Robtard
Prove it isn't!
Well, it's quite obvious, you have not proven that what makes lightsabers be able to cut through doors and what stops avadakedavras from going through doors are the same properties.
For a convoluted example, imagine a world in which ghosts can't go through salt (for example supernatural), yet they are able to go through doors, using a similar logic to yours that would mean that salt can go through doors, however that is not the case.
Or take a real life example, very similar, fire (a blow torch for example) can burn through steel doors, bullets are stopped by steal doors, yet a bunsen burner can not stop a bullet.
In the second case it is the time of exposure you would not have taken into equation (you could claim that lightsabers are instantaneous, but you'd have to prove that), in the first it is specific supernatural properties (though similar examples could exist in the real world) which operate differently (to disprove that you'd have to show that spells operate like other types of matter (which you can't) or you'd have to show that lightsabers can destroy magical or ethereal things (which you can't either).
Was that thorough enough?