Could a be a “facts and circumstances” type of legislation similar to the IRS. Many IRS rules go back to the “facts and circumstances” of a particular case to evaluate all the nuances relating to it.
I'm still not clear why you think this would be a good idea. Obviously you have opinions on what is "legitimate" in these situations. Can you explain more about this and how it wouldn't be a first ammendment violation to create such a law?
What "facts and circumstances" are we interested in? Your original post mentioned ending business relationships due to political stances. Did you mean the political stance of the advertisers, the platform hosting ads, some third party or parties, a combination of the above, or something I've missed?