It's not malware if it's done at the users request. And the user is free to display the web pages it downloads in any way they want.

It is nice analogy. Sophisticated malware disabled competitor's malware to be the one and only, acting like antivirus software.

About fifteen years ago, this was called Gator:

https://en.wikipedia.org/wiki/Claria_Corporation

You don't have a right to show ads to users. (The CFAA actually works in consumers' favor here: circumventing a user's adblocker is just as much of a crime as hacking into a server.) You do have a right to refuse service to users with adblockers, but to my knowledge Brave doesn't interfere with sites that do this - I still can't get past the paywalls on WSJ/NYTimes/Bloomberg.