I'd bet that channel "members" don't get ads for that channel regardless of premium status.
IMO, Google made premium, almost nobody bought it. So they went after adblockers, hoping that people would get premium to get rid of the ads. People most just Adblock harder.
While this is happening, one exec is peering over the fence at twitch. Where they only way to get away from ads without a pretty good Adblock, is to subscribe to the individual creator.
So they make "memberships" to channels a thing.
Almost nobody buys that either. So they go... What if, even if someone is premium, we give them ads, unless they're a channel member.
Genius.
Paying to block ads per creator/channel/whatever, is a special level of bullshit that twitch has always had.
The system is working as expected. The companies are trying to find the best way to extract the most value from you using their platform.