Individuals can have preference. Organizations with fiduciary responsibilities cannot. Also, the DNC did more than just have a 'preference' they conspired and acted against Sanders in an unfair and in some instances illegal way . Do you think organizing and training violent protestors is just showing 'preference'? Do you think busing repeat voters to different polling booths is showing only 'preference'?
Those actions from institutions from the DNC do more to undermine democracy in this country than Trump calling them out for that behavior.