I Thought America Was a Melting Pot--What's this About Illegals?

I'm hearing this term "illegals" a lot from my fellow Americans. It's meant to reference those who have entered the US illegally. But I don't get it. How can you call a person "illegal?" Does their very existence violate some human code?

And in America, the "great melting pot" that makes no sense. But then Americans tend toward reactionary extremism. We think patriotism means waving a flag or saying the Pledge of Allegiance. We proclaim to support our troops yet stand by while they have to fight for benefits. We know less about US history than immigrants. Our national holidays are just beer drinking blasts. We're fellow Americans only as it suits our purpose. We're ethnocentric while bashing each other's background (see this post I wrote).  Who Are These Illegals You Speak of? I Thought America Was a Melting Pot