If California bars all CA police officers for providing any cooperation with federal authorities on this matter, regardless of any "grant" the feds might offer to buy cooperation, then the feds will have a hard time enforcing the law. If California combines that with a vigorous "jury nullification" education program, then the feds will really be in trouble.
I don't think they want to do that. If they did that the proles would be refusing to convict each other for offenses on all the laws the WWICs (Whining Wussies In Charge) DO like.