Why are they suing Arizona?

The Columbia Daily Tribune of Columbia Mo. reports the following:

"JEFFERSON CITY (AP) — The number of illegal immigrants arrested by Missouri troopers has fallen significantly in the three years since the state began checking the residency status of everyone booked into jail.

The Missouri State Highway Patrol turned up an average of one illegal immigrant per day during the initial months after former Republican Gov. Matt Blunt ordered it to check the immigration status of everyone it incarcerated in August 2007, according to records obtained by The Associated Press.

But the number of illegal immigrants found by those checks declined by more than 40 percent in 2008 and an additional 30 percent in 2009, according to the patrol records. It has leveled off at that lower rate this year.
Missouri’s gubernatorial directive was expanded to all police by a state law that took effect in August 2008"

Ok, now tell me again, why is Obama suing the state of Arizona????