I've mentioned two reasons why it is wrong for a court to strike down an exception to a criminal law:

1) It is unfair to people who relied on the exception and are now converted into felons.

2) It is beyond a court's authority to create crimes that the legislature didn't want to create.

Let me add a third reason: It is simply beyond the court's power to make such a ruling anyway. A normal injunction in a constitutional case orders the state not to apply a statute. A court can enforce this injunction by, say, contempt of court if the state goes ahead and applies the statute at any point in the future.

But an injunction against an exception is essentially ordering the state to positively undertake prosecutions of the people formerly excepted, here retired peace officers. And the court has no effective authority to issue such an injunction, or to ensure that is obeyed.

This is true not so much for doctrinal reasons, but for practical reasons. Prosecutors have prosecutorial discretion. If they don't happen to prosecute any retired peace officers, what is the Ninth Circuit going to do about it? Nothing, that's what. There's nothing that it can do. Which is why issuing such injunctions is not appropriate.