They have a paid option. It's paying or ads. Running a search engine is not free. Somebody has to pay for it.
I mean, OP's not wrong. The phrase doesn't have to specifically mean an exchange of goods for money - can sell you on an idea, for instance. They "sold you" on the idea of using Brave by being very anti-ad. Now that you "bought it" and are hooked, they'd like some money.