Although exploitation undoubtedly happens (and not just to migrant workers) I don't think you can compare the situation in the US with the movement of workers within Europe, which has come about because of agreements reached by all the EU countries.

As I understand it, the illegal labour market in the US is a major problem but then again there are reasons for this:

1) It is natural for a capitalist economy to source the cheapest possible labour. If you don't like this fact, don't vote in rabid capitalists as your political representatives.

2) The lifestyle of promoted by everything American will attract people who are worse off and want to live the dream (unaware the American Dream is a sham anyway), and don't see why they should be denied it. This happens to a greater or lesser degree to all industrialised nations with reasonable standards of living, the problem for America is the cheap labour they've relied on for years has phoned home and now everyone wants a piece of the action.

It's going to get worse too. As the world heats up water will become one of the major commodities over which wars will be fought and as people's own supply dries up, mass migrations will occur in the search for water. You ain't seen nuthin' yet.