They are only "criminals" in the same sense that I am a "criminal" for watching DVDs on my PC using DeCSS -- because some asshat in office with a penchant for power decided to make something so silly illegal on paper. Crossing some immaginary line in the desert at night does not magically make someone a criminal in any meaningful sense.
Of course, the US is far from unique in this position, as all nations seem to attempt to enforce immigration policies. I've yet to see a truly compelling reason to support this practice. The free mingling of peoples and cultures is what's advanced our world to where its at today.