Andrew Sullivan puts it concisely:
Any non-citizen in this country is at risk of government detention at any time. If they can do it to citizens, they sure can to aliens.Without citizenship, they can throw you into jail and leave you there indefinitely with no legal recourse. It happens frequently to people who’ve fled violence in their home countries, overstayed a visa, or crossed the border to work. But since, as it turns out, the reason we have a legal system that allows things like this is because most citizens are A-ok with these outcomes, it’s unlikely to change anytime soon.