So let’s break this down.
The conservative justices on the Supreme Court dissented to the Arizona decision because they believe that it is the state’s right to implement and enforce its own immigration policy, separate from the federal policy.
Meanwhile, Montana wanted to enforce its own campaign finance laws for state elections, separate from the federal policy. This was struck down by the Supreme Court as an infringement on free speech, because money of course equals speech.
So in this court’s view, states’ rights are important, but not as important as corporate rights.