Same with things like Shambling Mound or Gelatinous cube (neither of which actually get to attack the monster inside of it, just do routine damage per round).
Why can't a Gelatinous Cube use its Slam attack against a creature which is already Engulfed?
I'm going to have to re-think this.
I can see the counter argument that it's the defender's thing to anoy monsters even when sitting in their gullet.