Durability does not keep it's edited state / no negative damage values

So all you have to do is:
1. Get something that takes durability damage when using it.
2. Using NBTEdit or MCedit change the damage value to -30000 (Or a little less)
3. Use it in-game and it does not keep the damage you set on it.
What I did:
I was wanting to change the durability on items so I went into MCEdit and changed the durability on a bow to be -30000 so it wouldn't break for a while. When I finished, I shot the bow, but it took damage. This should not have happened.