Hans von Spakovsky and Roger Gordon, Jr. raise that question here.
For all of the concern shown for the poor, misinformed shareholders who supposedly must be protected from the evil decisions of their corporate overlords to spend money on political advocacy, no one seems concerned about the members of unions, particularly those members in states without right-to-work laws who have no choice but to belong to a union. Their union bosses spend members’ dues on politics in much larger amounts than individual corporations do. And most corporate PACs split their contributions between the two major political parties much more evenly than unions.
The way all of the legislative proposals that attempt to reverse the First Amendment are being structured make it appear that partisan politics may really be at the root of what is going on.