This is not entirely true, at least not anymore. Twitch forces ads every two hours? or so nowadays (some predefined amount of time), even if the streamer has ads disabled entirely. I believe it's either that or preroll ads? Don't quote me on this but I've seen it happen and explained like that by more than one streamer.
Prerolls in particular are super annoying to the point that I've moved on to twitch turbo, even though I have a subscription to the vast majority of the creators I watch regularly. Just opening a stream of some other creator to see quickly what is going on and just getting slammed with a 30s+ AD is not a good experience.
In fairness to twitch though, they do cover all the bandwidth usage from all these streamers, including the ones which don't even qualify for partnership and ad revenue. I'm just glad they offer the option to pay to get rid of these things.