Firstly Bernie Sanders was supported by 45 percent of Democrats in the primary. All year long he has been the most favorably viewed politician in America.
He had got more votes from young people in the primaries than Hillary Clinton and Donald Trump combined. Hence he should be supported by the Democrats
Just because Bernie Sanders didn't win the primary doesn't mean that people should not still cheer for him and support him if they choose to do so. Bernie Sanders had a lot of ideas that people resonated with, and cheering for him simply validates those ideas. Democrats, and in fact all people, should continue to cheer for and support whoever they agree with.
The Democratic Party should be cheering for Bernie Sanders. Democrats need to be nice to the man that could make their convention chaotic if he is unhappy. Sanders has gained enough delegates to demand to speak at the convention, as well as appoint several members to the platform committee. Democrats need to keep Sanders happy so that he will support the party.
If you support him, and you are a democrat, then by all means you have the right to. I'm not sure if something happened with Bernie Sanders that causes democrats to withdraw support, but just like any other republican, supporting Trump or one of the others that have now dropped out, it is their right.