Why are immigration reform activists saying do not arrest Hispanics this could lead to racial profiling?

...I am a product of immigration, but nobody has EVER stopped me on the street and asked me to furnish proof of citizenship. Now why do you suppose that is? I think it's because I am white. My neighbors, on the other hand, are not so lucky. They are just as much citizens as I am, but they have to prove it to the cops because they are brown.