When Barack Obama and Eric Holder made the decision to challenge Arizona's SB1070 they did not consider the law of unintended consequences. While Judge Susan Bolton did temporarily block some some elements of Arizona's Immigration Bill she let stand ARS 1051(A) that removes the power of state officials, agencies and political sub-divisions that seek to limit enforcement of Federal Immigration Law. She also let stand ARS 1051 (C)-(F) forcing state officials to work with federal agencies on any aliens unlawfully residing or passing through the state.
Sanctuary cities create policies that instruct city employees not to notify the federal government of the presence of illegal aliens living in or passing through their communities. Most Sanctuary cities also instruct city employees to ignore immigration status (Legal v. Illegal) when they apply for city services.
In the same way that Federal law supersedes State law, State law supersedes a city or county policy/law.
Thirty two states now have one or more Sanctuary Cities and prior to Judge Bolton's ruling in Arizona there was little state officials could do about it without a contentious State v. City battle.
Judge Bolton has provided a legal precedent that can be used by other states to reel in renegade city and county officials that pander to illegal immigrants (of all nationalities) expecting Quid Pro Quo in the form of political donations and votes from minority groups seeking to become the majority, unions looking to grow membership dues, greedy business owners who want the cheap labor and churches wanting to fill pews and increase tithes and offerings.
I am sure there were lots of high fives in certain quarters when pundits declared that Judge Bolton had temporally blocked key elements of Arizona's tough new immigration laws but the elements she did approve are tools for state lawmakers across the country to end the Sanctuary movement.