The rule isn't that it's illegal to take money. The rule is that “unfair or deceptive acts or practices” are illegal with respect to advertising. Obviously that's a very broad stroke, so there are guidelines[1] put out by the FTC which give examples of what is considered fair and what's deceptive. In particular the disclosure should be part of the ad, not located somewhere else that a party might not have access to (e.g. YouTube videos are often embedded. You won't see the description box in that case, and will have no indication that you're watching paid content.) It's WB's product; they're the one's who needed to make sure their paid ads were compliant. Kjellberg/PewDiePie is based in Sweden, so the FTC guidelines don't concern him except insofar as he's doing commerce in the US.
[1] https://www.ftc.gov/system/files/documents/plain-language/bu...