Injunction Against "Don't Ask, Don't Tell"

The judge who ruled "Don't Ask, Don't Tell" unconstitutional has issued an injunction against enforcing the law. I applaud this, of course, and I have to wonder why the Obama administration appealed the original ruling. Since they have already asked Congress to repeal the law, why are they fighting for it in the courts? I suppose somebody could say that it is the job of the Justice Department to uphold the law, whether they like it or not. But in a situation in which 70% of Americans oppose the law, and the House of Representatives has already voted to repeal, can't they just accept the ruling and let the whole thing go? Isn't it easier to say to their military critics, "It doesn't matter what we think about the law, the judge said it's unconstitutional"?