There’s a federal grant program that gives local governments money for the cost of jailing illegal aliens, and some of that money is given to jurisdictions that deliberately avoid prosecuting illegal aliens. The Center for Immigration Studies notes:
In 2010, the grant program, known as the State Criminal Alien Assistance Program (SCAAP), doled out a total of roughly $400 million to about 850 cities, counties, and states. Among them were 27 jurisdictions widely considered to be sanctuary jurisdictions, which together received more than $62.6 million, or 15.6 percent of the total … . For example, the 2010 SCAAP grantees include some jurisdictions — such as San Francisco, Chicago, Santa Clara County, Calif., and Arlington, Va. — that are trying to opt-out of Secure Communities, the program that automatically flags criminal aliens for Immigration and Customs Enforcement (ICE) attention at the time they are booked into jail. [Internal citations omitted.]
Whatever you’re position on immigration—pro-, anti-, or somewhere in between—it doesn’t make sense for the federal government to hand local governments money for doing something that they do not in fact do.