Here's a pretty good answer, I think, from Wikipedia:

"The rights of both citizens and non-citizens within the U.S. are protected by the U.S. Constitution. However, there are some rights specifically reserved for citizens, such as eligibility for Federal jobs and to vote.

In actuality, the Constitution doesn't apply to "citizens," nor does it even apply to "people." It applies to the government. It tells the government what it can and can't do (the body says what the government can do, and the Bill of Rights says what the government can't do).

Immigration rules are administrative ones, and are mandated by Congress, not the courts."