I get that it's unethical do it without users consent,
but I am personally OK with anyone using affiliate links for whatever I subscribe to. I don't lose anything, I don't pay for it, some company is letting other people or smaller companies increase budget. I often ask my colleagues if they can get me referral link before I signup for something. What's wrong with it?
A widget hand-made by an owner-operator in small-town USA who sells from his own website gets ignored while the Amazon-listed widget made in a Chinese sweatshop gets shilled because Amazon pays affiliates.
This can be used to track users across the web without their knowledge or consent.
However I don't know Brave's implementation here. Just answering the question in the abstract.
It has a taste of paying too much if the vendor can afford to give a fraction to the affiliate.
Of course you pay for it, it's the same thing as free shipping: the seller increases its prices to take into account how much they're going to pay to affiliates.