How easy or even feasible is it to track where the money is coming from? Let's say you're a startup and a VC wants to invest in you. Can you ask for that information?
If you go public, you have zero control of who owns your company. It's not more ethical - it just launders your ethical responsibility.
There are plenty of people or organizations - who also happen to have a horrific ethical record - who can buy shares in a private company. And there's nothing you can do to stop that.
One must be prepared, of course, for the ramifications of asking a question and getting an unpleasant answer.