There are a lot of things that cannot be done anymore because of the negative weight in history. Ideas so abused that we dare not try them ever again.
Like Literacy testing for voting. Good idea to prove that the electorate can read and write, so that they have a good understanding of what the hey they're voting for in the first place, yes? Unfortunately, we've had a history of it being applied in a racist fashion. A white would-be voter would be given an extremely easy question, a black would be voter would be asked a question with no real answer. (Like "How many bubbles are there in a bar of soap?") Any answer he gave would result in him being told that he was illiterate, and he would be rather impolitely told to leave. Usually with a racial slur thrown in. So, it is now illegal to have literacy tests for voting, because we can't trust some people not to pull that shit again.
Same deal with poll taxes. Elections do cost some money. Have to print up the forms, collect the results, pay someone to count it, (and count it fairly dammit), booths, workers to explain instructions and make sure no one votes twice, and so on. So the idea being that paying to vote would recoup the expenses, as well as strongly discouraging double-vote cheating. Except that again black people were charged and turned away if unable to pay, but white people always got it mysteriously "waived." So, several court decisions later, it's not legal to do that. (And besides, it's kind of unfair to the very poor.)
There's a lot of ideas out there that might have worked out very well, but for historical reasons, are untenable today. We just can't trust people not to screw them up somehow.
So when people tell me about this law in Arizona, the one that where any "reasonably suspicious" person can be subject to arbitrary deportation, same sort of problem emerges. The technical language of the law may be reasonably neutral, but it's fairly obvious at this point that it will be enforced with "Latino-looking" substituted for "reasonably suspicious." Which kind of torpedoes the entire thing. Already 20 people with US citizenship have been deported. Without a chance to gather the paperwork that would allow them to, you know, return.
Mexico's not pleased about suddenly having a bunch of American refugees when they're already suffering a whole host of other problems.