Imagine for a moment if the Oshkosh Police Department decided it was going to stop enforcing drunk driving laws. No more extra patrols on Friday or Saturday nights. No more keeping an eye on the "bar area" or pulling over drivers weaving in their lane. Those stopped for other traffic offenses would not be asked "have you been drinking tonight?" Anyone who reeked of alcohol would not be asked to "step out of the vehicle".
How many crashes, injuries and deaths would have to happen before you were down at the Public Safety Building or at City Hall demanding that there be enforcement of the laws that are on the books? Would you think about forming your own Citizens' Patrol to try and bring some safety back to the streets? Well that is the situation the people of Arizona are dealing with when it comes to illegal immigration.
Their Federal Government is doing nothing to control the flow of illegals entering the state and it is doing nothing to remove the illegals that are already there. So Arizona lawmakers are taking matters into their own hands. Maybe it's only meant to be a wake up call to Washington: "we are sick and tired of putting up with more crime, violence and drug trafficking--start doing your job". Or maybe it really is a challenge to the seperation of powers between the states and the Federal Government.
Either way, you have US citizens demanding their laws be enforced--whether by Federal agents or local constables. I'm sure you would do the same if crime was happening right outside your front door as well.