Oddly, those people have a better argument than do the ones who wanted to vote against Trump.
In the case of Trump, the involvement of the Russians was reported long before the general election, so there is no argument that voters were tricked or deceived. They saw the emails, heard the arguments about the Russians, and cast their ballots with that knowledge.
In the case of Clinton, the machinations of the DNC weren't exposed until after primary voters had cast their ballots, meaning that votes for her were based on facts deliberately hidden by the DNC. So, the argument that they are not morally obligated to follow the pledge to vote for the DNC nominee is somewhat more legitimate.
Although it still sucks.