If Constitutional rights did not apply to non-citizens, then the government could: incarcerate or execute non-citizens without trial or legal representation, bug their hotel rooms and conduct warrantless searches of their possessions or persons at will, throw them in jail if a government agent didn't like something they said, torture them with impunity, etc.
It could be argued that all of the above is already happening, to non-citizens and citizens alike. But that’s another topic.
So someone tell me, what benefit is there to becoming a citizen if all Constitutional rights should be given to all persons that happen to be standing on U.S. soil? Maybe this is why no one bothers to become U.S. citizens?
Don’t get me wrong. I’m all for immigration. LEGAL immigration.