Corporations and organizations are comprised of MANY stockholders or members. The money that these people entrust to the managers of their groups is meant to benefit and reflect the best interests of ALL of them. There is NO WAY that a Board of Directors or Union leadership can ascertain 100% of what the people they represent wish in regards to political issues. It's guaranteed that _any_ decision in regards to campaign contributions or supportive campaign advertising will NOT represent 100% of the people whose money is being spent.
If a Board of Directors or Union wants to make political expenditures, require them to return that money to the people it came from, with messages urging that the recipients make the contributions directly.