I mean, why wouldn't they? Bernie is not a Democrat, he's independent. Winning elections for their own party is the whole reason DNC exists.

A primary role of political parties has traditionally been to filter candidates. It's literally their raison d'etre

Sanders wasn't even a Democrat. He switched solely to run in the primary. It's neither scandalous nor surprising that the DNC would try to put up barriers between him and the nomination.

If the RNC had done it's job, Trump would never have been allowed into the primary in the first place.