Because that's how it starts. Eventually, the cheaper tier will be with more ads, the more expensive tier will no longer be ad-free, but have "some" ads.
HBO has been ad free since the 70s. There hasn’t been a cable service or streaming service that started ad free and then didn’t offer an ad free offering
All cable networks started ad-free because they charged a monthly subscription fee, then almost all of them gradually started introducing ads in addition to the subscription fee. HBO is one of the few exceptions. The fact that cable networks let you pay a second monthly subscription fee for HBO hardly constitutes an "ad free offering."
Where does this myth come from? Cable was first used to bring network broadcast TV with ads to places with no reception. Then came the “Superstations” like TBS and WGN. That were rebroadcast of local stations over satellite. Then came ESPN, CNN etc. cable TV always had ads except for the premium channels.
It has happened before. It will happen again.