It isn’t actually, not in all cases. There is nothing in the constitution preventing it and the Supreme Court and state courts have said that there is no mechanism in place to either identify it objectively, nor to remedy it if found, with a few exceptions. The biggest exceptions are where it violates the Voting Rights Act or otherwise demonstrably discriminates on the basis of racial demographics, in which case it can be kicked back to the legislature with the directive to try to be less racist this time.