Why does negating part of the damage disable stealing costumes?

[Acewhatever]

Both editions of Halloween rules state that you need to roll at least 3 claws to steal a costume. It infers that the monsters need to be damaged but not specifically by 3 points, so it should stand to reason that you don't need to deal 3 damage to steal otherwise stealing would become much more difficult with all the damage negation cards available.

[voriki]

It doesn't. When part of the damage is negated(example Robot Costume), but all, then you can still steal.
You can even steal from indirect opponents if I am not mistaken.
Player A is in Tokyo, Player B rolls 3 claws and also has the card Fire Breathing(damage neighbour with 1 damage). Player A gets damaged by 3 claws+1 damage by Player B. Player B also damages his neighbour player C. Player B is allowed to steal a costume from player C if he wants to as the damage occured simutaneously with the rolling of the claws.