How does Mexico treat immigrants?

With "abuse, torture and extortion".

On Oct. 26, Yanisleidys Pineda Nápoles turned 20 years old. Her birthday gifts — the only items allowed to enter Iztapalapa’s Migratory Station — were a towel, a tube of toothpaste, a brush and a roll of sanitary paper.

She has spent six months in Mexican migratory prisons and, even though she is no longer deportable since she left Cuba more than a year ago, she doesn’t know how long the Mexican authorities will keep her in detention.

Hers is not an isolated case. In the past two years, after the Memorandum of Understanding between Cuba and Mexico was signed on Oct. 28, 2008, the passing of Cubans through this country en route to the United States has turned into a riskier and more expensive adventure.

Non-government organizations, such as Amnesty International, Without Borders and the Cuban-Mexican Civic Association, keep records of abuse, torture and extortion of Cubans by the Mexican authorities of the National Institute of Migration, the Navy, the Federal Patrol Police, the District Attorney’s Office and even the federal, state and city police, not to mention kidnappings by organized crime.

http://cubafaq.wordpress.com/2010/11/15/cuban-migrants-report-abuses-in-mexico/

That's rignt. The Mexican government which sued Arizona to enjoin police officers from simply asking arrested criminals for identification in turn "abuses, tortures and extorts" immigrants in Mexico.