From your description I don’t see how businesses and unions are doing anything different; they’re covering their business interests.
I don’t agree with the unions approach but they are no more culpable than are business interests.
What the corporations and the unions are doing are completely different. The businesses are donating to both political parties, usually pretty much equally, as a matter of business survival. It's not an option. It's a necessity for them, just as a neighborhood business in a Mafia controlled area has to pay protection money to the local Mafia thugs, to avoid getting their business or theirselves busted up.
On the other hand, unions take their members dues, some of whom vote Democrat, and some of whom vote Republican, and all the union political donations go to the Democrat party. If they donated part of those dues to the Republican party, Harry Reid is not going to break the knee caps of the union leadership, or send the Justice Dept after them, as happened to Microsoft, when they didn't donate "enough" to the political parties. So what the unions are doing is not at all the same as what corporations are doing.
If I were a Republican paying dues in a union, I wouldn't particularly like the union's practice of giving my portion of dues that are donated politically, to the Democrat party, nor would I feel that it is "fair", though there would be nothing I could do about it as long as I chose to stay in the union.
For myself, because unions support the Democrat party exclusively, I chose to do business with non-union companies, whenever practical, since my choice is to vote Republican, and to try to avoid supporting the Democrat party in any way.