Unions often don't represent the beliefs of their members, they usually represent the beliefs and goals of the highest ranking members (and sometimes the people that pay them off, unions are not exempt from corruption). Sometimes the goals of the leaders line up with the goals of the majority, sometimes not. When I was in a union, many items were decided without a vote and no effort was put in to informing new members on how to be represented within the union.
I should also add I don't think unions should not be involved in politics; I think they are used as crutch for democratic representation. The real solution is removing corporate lobbying.
I should also add I don't think unions should not be involved in politics; I think they are used as crutch for democratic representation. The real solution is removing corporate lobbying.