Not sure it would apply in this case since they are suing city governments, but interesting point.
Yeah, Protection of Lawful Commerce in Arms Act isn't going to help a city that's not selling arms when somebody sues them to try to force them to pass a law.
This lawsuit is stumbling on new territory, where nobody ever thought a plaintiff's counsel would be stupid enough to tread, so there aren't any laws aimed at it.
IANAL but I think cities should motion for dismissal on grounds of frivolity and ask for sanctions to cover attorney fees.