Sumpremacy

One thing I don't get. If it is a violation of the Supremacy clause when a state enforces federal law, why is it not a violation when a city declares itself a sanctuary and refuses to allow the enforcement of federal law? Moreover, isn't the Arizona law simply a set of guidelines as to when the suspect is referred to the federal authorities? The decision to deport or what constitutes a deportable alien is still entirely in the hands of the federal authorities.