AZ's controversial law re police and illegal immigrants before Supreme Court

Oral argument heard today in D.C.  If the Obama administration was serious about controlling illegal immigration from Mexico, wouldn't it welcome, rather than challenge, the AZ law?.  The AZ law does not interfere with federal law or federal law enforcement. It's purpose is to assist these.  The fact is there are many on the political Left and Right who really don't want effective control of illegal immigrants. One of the many "let's pretend" (e.g. gun control, improving public education) games played by politicians Why doesn't the administration challenge cities that promise to offer help and "sanctuary" to illegal immigrants?