My first question is, "why not?" I think arresting illegals should be something every law enforcement agency, local, state and Federal should do. There's a sheriff in some county in Ohio who was Fox & Friends who is arresting illegals and their employers. It can be done. One thing that's good about Sensenbrener is to REQUIRE it by not funding any State or municipality who refuse to enforce immigration laws.
I misspoke. They can arrest them they just can't make it stick because of the language thing. Judges keep throwing the cases out. Consequently, they are discouraged and not bothering any more. If they stop someone and the person speaks another language it pretty much "so long amigo."