I think it is probably something along the lines of the announce table, ring steps, barricade, etc. are all part of the ring/arena, so throwing them into the ring is just like slamming them into the mat. Weapons introduced from under the ring, tights, etc. or that have been banned previously are illegal, thus a DQ. I think introducing a weapon would be considered "illegal" (which explains Michael Cole's spasms), but using it indirectly (Van Daminator, DDT, etc.) is legal. I would assume bring a chair in to DDT your opponent on is illegal, but DDTing them when it has been brought in by the opposing party it is legal.
