This isn't true for all streamers, just those with partner contracts from Twitch. They are the ones that are required to run ads a certain amount, I think below that they have the option to run ads but aren't required to but also if you don't run any ads Twitch will just start putting in ads on its own. And theres no control from the streamers over ads that show when joining a stream, those are always there and in my opinion are more annoying cause it makes it more annoying to switch between streams.