And how does that negate people's ability to choose the products they will buy or keep companies from catering to specific portions of the market and thus caring very much how people vote with their dollars?

Geez, do I really have to spell it out for you? Politicians have to somehow get a majority vote. Voting with dollars would only requires one rich person to support a company, it doesn't matter if no one else pays them a dime. It's sort of the extreme inverse intention of most campaign finance laws.

I still can't see a problem here. A business with a single customer exists in an almost complete vacuum and doesn't influence the market. What's more it's very risky as its revenues aren't diversified. It's completely dependent on its only customer.

that's right. consumers can have a monopoly over a business too. in fact, I seem to recall there are a whole bunch of companies struggling to figure out how to please the customers of a single niche and that live or die depending on how well they do that.