This is a completely bogus legal argument that seems politically motivated to me.  I read the opinion and it's not logically consistent.  For example, compare Hawaii Revised Statutes on speeding vs Maui County Code.  Exact same subject matter, plus the state intended for the traffic regulations to be comprehensive and "uniform throughout all political subdivisions."  This is a direct and obvious example of Maui County regulating something which this court ruling claims they can't.  

What makes this more complicated is that anti-GMO activists are COMPLETELY wrong on the issue of genetic engineering (there are no pesticides unique to GMOs, thus no unique health dangers).  So while they are wrong on the issue, this court has a very bad argument.  But after this election I've realized that my pro-GMO friends are no more rational than any anti-gmo activists that I've ever met so they will just ignore the evidence and my argument just as the anti's do (and for the same reasons).