I'm not a police officer. But even I know that many states don't require front license plates. How do I know? Because I've seen many cars with out-of-state license plates without them.
So when a police officer sees a car with Florida tags and stops it because it's "missing" a front license plate, that's not a permissible stop. As virtually every other court in the universe has also held.
It doesn't seem overly burdensome to me to require officers to know the same laws that even regular citizens can figure out. Even if (as is not, in fact, the case) ignorance of the law was an excuse that permitted officers to violate the Fourth Amendment. Seems to me that you could go over the laws of the various states about front versus rear license plates in five minutes or so when you're training someone about the other traffic laws they need to know. They could even write it down and take it with them in their cruiser. So I'm not really sympathetic with the contrary position of the Calfornia Attorney General here, which I think the Court of Appeal properly rejects.
Now, mind you, I don't think the officer was the only one who made a stupid mistake here. The driver probably shouldn't keep his cocaine in his sun visor either, at least if that's where he keeps his registration papers too. Seems pretty common sense that you don't want your drugs dropping down whenever you have to pull out your registration. Cops take a dim view of that.