Unions are accountable to their memberships as a whole, as every member has a vote, thus making it similar to every individual within that union donating that amount of money. In a corporation, the only voting rights belong to the wealthiest shareholders, thus enabling millionaires and billionaires to donate money from their companies without any accountability whatsoever. It’s really a matter of who can observe the actions of the donating body and who can do something about it. If a union is doing something the workers within that union don’t like, the union heads get voted out. But if I work for a corporation run by someone who just spent $10 million of our company’s money to give to a presidential candidate, there’s nothing I can do but hope that money isn’t coming out of my pension.