Wait, why should brave get to throw its own ads on example.com? If the little fish and big fish were swapped, would you say the same if Chrome had an [opt-in] box for replacing all non-google ads with doubleclick ones?

I wasn't clear. Both are unethical in my view, even if one is widely practiced.

No, but you can always ignore those ads or not click on the sites that serve them.

But in the case of Brave, they are essentially modifying the producers content without their consent.