I'm not sure why everyone is talking about enforcement of laws (other than the incorrect thread title of course.) The issue is whether or not attorneys general need to argue in court on behalf of laws they believe are unconstitutional. For instance, the Holder DOJ believed that DOMA was unconstitutional and didn't argue in its favor before the Supreme Court. They still enforced the law, though. There is a big difference between those two things.
Homer: [looking at watch] Two hours? Why'd they build this ghost town so far away?
Lisa: Because they discovered gold over there!
Homer: It's because they're stupid, that's why. That's why everybody does everything.