A lot of people view corporations in a justifiably negative light (very few people say that corporations are an unalloyed good, and I have never heard anyone wish that they could work for an LLC). But a corporation is chartered to make money - we expect it to act accordingly and for the most part companies do make money (when they fail to make money, they cease existing and the general consensus is 'good riddance').
A union on the other hand is chartered to advocate for the rights and needs of workers. The standards and expectations for a union are fundamentally different than for a corporation. It depends on the union, but a lot unions don't really carry out their core function and there are no consequences for this failure. Given that Unions claim a democratic mandate to advocate for labor, people are justifiably upset when they see their union doing nothing to represent their interests.