MoveOn.Org has always been a very liberal movement and organization. I think that Bernie Sanders policies and ideology fits the far left better than does that of Hillary Clinton or any of the other Democratic Presidential Candidates. Because MoveOn.Org is viewed as very left wing in its bias, purpose, and identity. I think that whether MoveOn.Org's officials like it or not they are expected to endorse Bernie Sanders.
MoveOn.Org has helped many individuals be connected to millions of Americans to help win campaigns. This is a useful tool for Bernie to receive more recognition and support from a community that cares for progressive change. Bernie has a very different look on how the United States of America should be run, and could use the proper endorsement behind him to help him into office.
Why should Sander's campaign not have been endorsed by MoveOn.Org? If the organization wants to back Sanders they should. The DNC clearly wants everyone to fall in line behind Clinton, but that's the problem with politics today. The people and organizations should back who they want to back. They should not feel forced one way or another.
As one of the most leftist organizations in the United States, it is perfectly appropriate for MoveOn.Org to support Sanders' candidacy. MoveOn appears by all accounts to be better aligned politically with Sanders and his far left socialistic tendencies. MoveOn has some political clout, and Sanders should welcome the endorsement.