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.
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.