Why does San Francisco say they cannot arrest illegal aliens as they might snare one wanted minor crime like?

Why does San Francisco say they cannot arrest illegal aliens as they might snare one wanted for what they say is a minor crime like, drunk driving or driving with no license / insurance, therefore they now refuse to arrest illegals, thus allowing them to break minor laws as above with no consequences, serious criminals in the country illegally, whom have committed serious crimes like murder, kidnapping, drug & human smuggling , MS 13, Mexico Mafia all cannot be arrested because it might catch that one illegal who has an outstanding ticket , so why is San Francisco now telling the world, there are two sets of laws one for illegals and another for Americans ? Would the Mexico drug cartel be welcomed in San Francisco and be granted sanctuary to save that one illegal from getting arrested ? Why is Obama allowing sanctuary cities to make their own immigration laws as long it is for benefit of illegals Obama refuses to do anything about it ? Obama praises sanctuary cities and condemns and Sues states like AZ and allows all others to make their own laws regardless if it is against federal laws just as long as they offer sanctuary to all illegals ?