Senator Barack Obama makes life difficult for Mexican Guest Workers in US

How can Democrats explain that they are on the same side as the Mexicans coming to the US? Isn't it clear to everyone that this provision does more to protect union workers than it does to help guest workers to make a living.

an amendment by Senator Barack Obama, approved by voice vote, extended Davis-Bacon wages rates to all private work performed by guest workers, even if their occupations are not covered by Davis-Bacon.