"Under federal law, union members can fill out a form to opt out of having a portion of their dues go to political activity if they disagree with the organization’s politics. Corporations, on the other hand, do not have to disclose to their investors how they spend money for political advocacy and lobbying, let alone let investors have a say in which candidates or causes the corporation should support. So investors cannot opt out because they don’t even know."
Why unions are not super PACs - Salon.com

The problem with your information is the fact that billions have been contributed to super PAC's by just FOUR of the worlds richest men while those union contributions are small individual contributions. Those four certainly expect something in return for their billions, those union members don't expect much. Unions are not even close to the the contributations made by less than 10 of the super rich.

Our
 

Constitution was not written for the benefit of corporations. They are not people and have No rights except for what the right wing has awarded them. They have such large monies and a bought and paid for congress with Supreme Court thrown in. The average voter doesn't have a chance in this rigged society. We are imploding while the greedy rich bleed our society for even more. It's disgusting!

Here are Top 20 Individual Donors to Super PACs in 2012:

https://www.opensecrets.org/outsides...&disp=D&type=V

Here are Top 20 PACs by Total Expenditures 2013-14. It's instructive to click on the PAC name and read more about its cause, if not recognizable by name.

https://www.opensecrets.org/pacs/top...=2014&filter=N

Use
 

That same site for reference. Change the parameters to be superpacs only. There is where the koch brothers hide their money's in many relatively small amounts with name changes to those superpacs. Just an attempt to distort the truth!

And it's where Tom Steyer hides his, in the #2 PAC "NextGen Climate Action", and League of Conservation Voters, for starters.

https://www.opensecrets.org/pacs/sup...php?cycle=2014

"NextGen Climate Action Committee is on pace to spend over 50 million, aims to knock on 1 million doors—and is touring Florida in an ark….."

Tom Steyer's Green Super-PAC Has Spent a LOT of Money on This Election | Mother Jones