I don't know much about US politics but I don't think they are determining who is on the ballot are they? They're determining who can claim to be the official Republican Party but I don't think there's any reasonable grounds to say that that's wrong.
Ted Cruz can still be on the ballot if he wants to can't he? As an independent? How could the Republican Party stop him?
There's "sore loser" laws in some states that would be a problem, and he would have to collect thousands of signatures in most states where he wants to be on the ballot.
It's a complicated morass of rules that would cost a lot of money:
If they had that role removed, they would still be able to endorse a candidate or agree to all vote for the same candidate or whatever.