Because it's monetizing other peoples content. In my view, it's the same as cutting and pasting somebody else's content onto my own site with my own ads.
Even if I promise to give most of the money I make back to the original content creator, I really have no right to the small part I skim.
Update: Sorry didn't mean to suggest it was unethical for you to view a page any way you want, but I think the creators of the Brave browsers are scuzzy.
Update Two: I see now that Brave have updated the policy and will only collect funds for creators that have opted in.
