There is a difference between offensive actions inside of a defensive war and pre-emptive war. Pre-emptive war is, by definition, not defensive. It initiates the acts of aggression against another country. Again, I'd bring up the analogy of shooting a neighbor "pre-emptively" because you think that he might be a threat to you in the future.
A pre-emptive strike, by definition is an offensive action. That does not mean that side that launches it is involved in an offensive war.
Your example does not cover the point in question. Nobody is arguing that every pre-emptive attack is moral. The argument is simply that they are not all necessarily immoral.
Let's say that N. Korea develops its nuclear weapon program to the point where it can get a warhead small enough to fit in an artillery shell. Furthermore, let's say that N. Korea decides it's going to take out the South, paving the way with nukes aimed at Seul. Continuing on our hypothetical journey, let's say the Allies have thoroughly penetrated the NK communications grid and has good human assets in key places. All of these report on the NK war plans. Normal recon shows the mobilization of the NK military taking place. A couple highly placed NK players defect to the south because they are horrified at the human slaughter about to be unleashed.
While there is a chance we might be able to stop NK planes or missiles armed with nuclear weapons, there is 0 chance we are going to defend against nuclear artillery shells. But our intell shows those nuke shells are still in a depot, and we have the time and capability to attack it before they are moved.
According to you logic, the only ethical choices here are for S. Korea to surrender unconditionally, or for S. Korea to accept millions of dead civilians in the opening minutes of the war because it is somehow inherently unjust to launch a pre-emptive attack.