Who decides? Why?
From America's beginnings, there were struggles between "us" and "them." First, it was between Columbus and the Native Americans. Columbus and the Spanish conquistadors were more powerful than the Native Americans. Their power, combined with disease enabled them to conquer the Native Americans. Columbus and his Allies were immigrants to America. This began a flood of immigration to America which never ceased. People from all over the world came to America either to make a better life for themselves or because they were forced due to slavery.
Either way, they were immigrants. Slowly but surely, America was becoming more and more populated which led to problems. At that point, people began to decide who was allowed to enter. This began the immigration debate. The irony of immigration is that most of us are ancestors of immigrants, yet we see ourselves as "true" Americans.
My opinion: Just because America is more structured than it was in its early days does not give any one group the right to decide who can enter. We all came from somewhere, and most likely our ancestors came from another part of the world. It is unfair for us to decide. The entire world is here for all people, animals, and plants. Not just for those who are more powerful. It is unjust that people are excluded based on their birthplace alone.