Because this story is an oversimplified analogy, propogated by people who like, even NEED, easy answers, or their worlds fall apart. The issue is far more complex than this story, but whatever makes ya happy. It's nice to know the lines between right and wrong can be so easily drawn to include those who think like you on the RIGHT side of the line, and everyone who opposes on the WRONG side.
And I DON'T believe that there's an excuse for illegal immigration, but the solutions thus offered are poor attempts to address the symptoms, rather than the disease. I don't pretend to have easy answers, so I'm not going to get into a "Well what would YOU do, smartypants?" flame war, but people who write passages like the one above seem to KNOW pretty darn surely WHAT to do, which is exactly why I won't listen to people like that for very long.