DailyRich, I totally agree with what you're saying. It is very thematic. It makes sense that I can't play Rebel Assault against it. I, too, thought that you can damage it with blast damage because, after all, it says it can take 10 damage.
However, when I saw this thread, I went to the rule book to see how wording is different between the different effects. All along, everyone says that you can't do anything to the Death Star dial except engage it because it is not an objective, but let's say you engage it. Okay now what? Congratulations, LS player, we're now having an engagement at the Death Star dial via Trench Run. What are you going to do next? Target of Opportunity? Nope, the Death Star dial isn't an objective, try again. Blast damage? Sorry, it's still not an objective. Unopposed bonus? Well, hehe, the dial is not an objective, and it most certainly is not a card. Guess that was a waste of your units' activations.
What I'm saying is that I agree with you on the intent of the card, but when I try to make the cards and the rule book tell me why blast damage works but Target of Opportunity does not, I'm coming up short.