It makes sense when you realize that the process for parties choosing candidates is not outlined in the Constitution or anything. A party doesn't need to hold a primary if they don't want to, they could just present a candidate.
If/when a party decides to have a primary, they can pretty much do it however they want.
This happens sometimes with third parties. For example, I don't think Ross Perot had to run in a primary. Nader may have (?), but if so it was probably mostly just a formality by the Green Party.
But yeah, I would probably prefer more regulated primaries, assuming it wouldn't just be used to fuck over progressive candidates, which it probably would be designed to do