The dividing line is privacy invading third-party ads.
At a whole 12M users, they're hardly extorting anyone at this point. Let's back the hyperbole train up a bit.
They're only free to do so in Brave's eyes if they roll their own ad solution, which is an option but not _always_ the best option. The "easy" (and, arguably, more featureful/secure/"better") route to monetize across the Internet (especially on hobby projects and small businesses) is third-party ads. There's lots of reasons someone might weigh the options and make the decision to go with a third party ad provider; Brave takes that decision away. Whether that's better or worse for the Internet as a whole is debatable.
wrt third-party ads being more secure... https://en.wikipedia.org/wiki/Malvertising
You also point the arrow of causality in the wrong direction. It's not as though Brave magically shows up on devices. The current ecosystem and its host of problems (including privacy and security) are precisely why users are choosing adblockers of all stripes.
In Brave's case, at least they're conscientious enough to provide an alternative path to monetization for creators.
Or they can choose block 3rd party ads and tracking scripts and opt-in to seeing ads they are more likely to be interested in and get paid a percentage to do so.
Chrome—and by extension Google—does not give you that choice and certainly doesn't share any of the ad revenue with web users.
It's not a bad idea, but IMO it's a bad execution.
They're effectively running a business based on extorting everyone building anything for the web (everyone from big businesses to small hobbyists trying to make something new) by flat-out replacing the most common form of monetization (ads) with their own monetization and saying, "Hey developers, I know you worked hard on that website, so if you want that money (or, at least, a cut of it we feel is fair), sign up for our platform instead".