I don't believe they can be prosecuted under 1324, unless, the Gov. can prove that they are taking overt action to shield illegal aliens from apprehension by the Feds, such as providing housing for them or employment or transportation, knowing that they are illegal. The cities cannot be compelled to enforce Federal Law for the Gov. It is the equivalent of a citizen refusing to report a crime to the police. The citizen is not required to report a crime by law and cannot be held liable for not doing so.