That's not true at all. At least on the Democrat side. The party chooses the nominee, not the people.
"The DNC, led by Wasserman Schultz, Admitted In Court they Rigged Primaries Against Sanders"
>A Federal Judge dismissed the lawsuit after DNC attorneys argued that the DNC would be well within their rights to rig primaries and select their own candidate.
http://observer.com/2017/08/court-admits-dnc-and-debbie-wass...
"DNC Lawyers Argue No Liability: Neutrality Is Merely a ‘Political Promise'"
https://observer.com/2016/10/dnc-lawyers-argue-no-liability-...