Why does an imaginary line in the sand determine a person's worth?

“What I have a problem with is them tapping into the resources, be it minimum wage/cash only jobs, school aid or whatever, when some Americans can’t get the help they need. We’ve got ppl in LA, still living in trailers after Katrina. Those ppl should come first.” “But, I don’t feel it’s my responsibity as a taxpayer to help support ppl that come into this country illegally.”

These are a few quotes from the immigration thread. Where does the difference between an illegal immigrant and a Katrina victim, or you and I for that matter, lie? Why does it make a difference if tax money goes to help people on “our” side of the line, or to provide care to an illegal immigrant with cancer? From our perspectives both Katrina victims and illegal immigrants are the same, both are strangers and human beings.

So, I guess my real question is, why does an imaginary border determine worth?