Earlier quoted context omitted.
https://en.wikipedia.org/wiki/Duverger%27s_law In light of that, if you don't believe parties have a duty to be fair to all primary contenders, then I'm at a loss. How do you expect to avoid cronyism and corruption when "private" political parties are outed working to bias primaries and you say that's okay?
what does "fair" mean? A party's purpose is to advance a set of ideas. So, absolutely, the party leadership can choose not to be fair to a primary candidate that does not support those ideas. As an example, if Sanders is a socialist and wants to advance socialist ideas, he can form his own party and have it be on the ballot. And that party should be free to reject primary candidates who are not socialists, no? otherw…
That's the entire point.