The left considers loyalty to be a vice. Bernie Sanders doesn't think he should be loyal towards his supporters, because by doing that he would be bigoted towards people who are not his supporters. He views his supporters as people whose interests are temporarily aligned with him, and does not feel he owes anything to them.
He has no concept of betrayal, because he views betrayal as the obvious action to take. He does not think they should feel betrayed by his willingness to throw them under the bus whenever necessary, because that's what you do.
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