That rule is a Democrat party rule.
What the Bernie bullies keep forgetting is that they're--or past Democrats like them--the reason for the creation of superdelegates, professional Democrats for whom the long-term future of the Demcorat party is their main concern, and hence add balance and sanity to the selection of Democrat candidates.
If the Bernie bullies--or past Democrats like them--hadn't been so willfully reckless in their selection of extremists as Democrat candidates who tended to lose elections because of their wanton fringe-ness, superdelegates would've never been necessary.
They got only themselves to blame.