What is the difference between “legal” and “just for the fuck of it.” The legal system is just a process. The facts, justice, etc don’t factor into it. As long as those states follow their process, removing a candidate from the ballot for any reason whatsoever is perfectly legal.

After all, you said that “state control over federal elections has always been fundamental to US federalism.” So why does a state need to allow any given candidate onto their ballot?