Corporations, like unions are free to campaign independently. For example the for profit Fox, Breitbart, New York Times and Washington Post Corporations all made their political preferences very clear over the past two years. They each spent a lot of employee time and money supporting their favoured candidate. Or Disney, a for profit corporation, releasing Michael Moore films. They’re political propaganda, just like the garbage Citizens United produced to denounce Hillary.
Political speech rights in the US belong to individuals, who do not lose them by coming together in corporate organizations, whatever their form. This is just.
There is an absolutely massive difference between corporations and unions, in that corporations have a fiduciary responsibility to seek profit.
Unions don't.
This is why Unions should be allowed to campaign independently, but not corporations.
If all of the shareholders decide to contribute with their dividends, they are already allowed to, which gives them an advantage. That ought to be more than enough.
Moreover, unions are legally prevented from contributing to political campaigns, while corporations are allowed to do so.
Therefore, the comparison breaks down at every single level, and no useful conclusion can be reached from it.