> Advertisers could push back on the notion that "an ad == approval"

Ads are far more than just "approval", they're funding. An ad appearing with a video is funding the production of more, similar videos.

It's quite meaningless to "boycott" a service that you're not paying money for, but people recognise that the modern flow of money around the economy is extremely indirect. This results in a lack of moral accountability. Campaigners aim to collapse the distance between those spending the money and the actual actions that result, to say "are you really happy paying for this to happen?"