Using nukes against the Mallers/Colony is a desperation move, there are much easier methods for raining violence upon them if needed. And if the entire country is over-run, what's the point in nuking zombie concentrations in LA?

On the other hand, if Ground Zero is the sole source of the global outbreak, or if LA is the unique home of the non-standard zombies and you can identify the "factory", then you might have a target worthy of nuclear escalation.