"Then the same rules should apply to an individual who has access to union dues."
I've not commented on whether unions should be able to make campaign contributions. The topic is corporations.
"... its assets are held to be used only in acceptable ways to advance that purpose,
That's your opinion, but not the law. I suggest you contact Ben and Jerry's and tell them that their donations to civic causes are not allowed because they do not fall within the scope of "ice cream business"."
Can you quote Ben and Jerry's purpose as stated in their corporate bylaws? I bet you can't. I also bet the phrase "ice cream business" doesn't appear in there anywhere.
So setting your non sequitur aside, no it isn't "my opinion", it's a fact of law and has been enforced in the past by shareholder lawsuits.
A corporation does typcially have a certain amount of lattitude. Charitable efforts, to the extent that they don't financially cripple the company's direct efforts towards its state purpose, are generally accepted as an attempt to support the business through PR and buliding good will.
And certainly it is true that campaign contributions can support a corporation's mission; hence the importance of the other fact you seem to mistakenly believe is my opinion:
"... and the government has every right and interest in constraining what ways are acceptable.
Again, your opinion. In my opinion, the government has no right, and indeed no Constitutional authority, to tell me that I cannot spend the corporate assets of a company I own in any way I see fit"
Nonsense. Let's start with the obvious: the executivies making decisions about campaign contributions do not "own" their respective corporations.
Second, what you're claiming is that if you contribute some of your assets into a corporation, you should get all of the benefits the government offers you as part of that deal but should still have full personal control of your assets, which is bs on its face.
The authority the government has to regulate a corporation's actions (above and beyond those of individuals) comes from the deal that a group of individuals made with the government in forming the corporation; those are the costs for liablity shields, special tax treatment, etc. without which those corporate assets (which are not the same thing as your assets even if you "own" the company) would never have been amassed.