When Has Enforcing the Law Become Such a Big Deal?

It's amazing how people are getting their underwear in such a bunch over an Arizona law that would actually enforce federal law, with Arizona lawmakers being called rednecks and racists. How can it be backward to enforce the law? If my memory serves me correctly, the rule of law is what always separated civilized nations from backwards nations. This truths goes back to Rome and even further. It is the sign of a banana republic when laws are considered arbitrary and not worth the paper they are printed on. Granted, there would be laws that are immoral and that shouldn't be enforced but it seems to me a simple one like "are you supposed to be in this country" is a no-brainer. Yes immigration reform is necessary but before then, I don't see what is the big deal about enforcing current law. And it's not like illegals don't already know to avoid the authorities, unless they are really stupid and just asking to be deported (so the argument that more crimes will go unreported doesn't have much to stand on).