This is an extension (and cause) of the Pen users' Ripple-Effect-Proof Memory - they actually have a Ripple Effect Proof Existence. If it was attempted, then history would simply warp itself around the changes in such a way that the target still exists. This is why Irving always just tries to knife Ethan instead of stabbing him through a Hole into a point before Ethan became a Pen user. There are a couple of holes in this theory which are easily fixed up with a closer look: