Hillary's supports seem to be taking Bernie's supporters for granted, and her donors are no different. If she wants to defeat Trump, who has a very real possibility of winning the election, they'll need to work hard to ensure that all of Bernie's supports are firmly behind Clinton come election time in November.
If they want to support Sanders supporters I don't think there is any reason why they can't. It is up to the people who have money. Although I think the whole companies especially media companies getting involved with donations in the elections in the first place is wrong and should not have been allowed.
Bernie Sanders in not the Democrat party's presidential nominee. Anyone who donated to Hillary Clinton should be able to know their donation is not being used to support the candidate they opposed in the primaries. Donors should not have their support misappropriated by the party. Bernie Sanders is responsible for obtaining his own donors.
Like it or not, Hillary Clinton is the nominee for the Democratic party. The best way to ensure a victory in November is for all members of the party to rally behind their candidate. However, once the election is over, Clinton donors should look at some of the specific issues Sanders supported, and determine whether they want to make contributions to them. But making donations directly to Sanders supporters is a little now.