But I still wonder why the government went after them with such a vicious vengeance when, as you pointed out, "benign neglect" had been working just fine for the open-borders crowd for decades.
I think the answer is us - they are concerned about people like us. As long as illegal immigration was mostly a border-states issue, and mostly confined to a few neighborhoods and a few cities, it was tolerable, and the voices against it could be ignored. Now illegal issues are nationwide, visible, and problematic.
But most importantly, now there are many, many cranky, outspoken Americans who are openly questioning our government's "immigration policy". I think that's the big change.