Trapper thieves are able to lay traps as a ghost, which is a very exploitable bug. As a ghost, they can take all the time in the world to lay traps all over someone's cabal exterior, with zero risk to themselves, and then when they reform the defenders will hit all the traps, becoming blinded etc. The fact that the traps do not hit anyone until the trapper is no longer a ghost suggests the intention was that traps laid by ghosts are not harmful to the living.

You can argue that it shouldn't let them do that, but it currently is intentional.