Apple TV certainly has ads. It’s just that it’s ads for Apple products.
If you do, I suppose what I would amend my statement to is: it doesn't show ads for apps I don't explicitly select in the UI. Either way, that's much better than most competing products... And it's incredibly fast, with the lowest latency of any streaming device.
I don't like Apple's locked ecosystem, and avoid most of their products. But the Apple TV is just head and shoulders above anything else on the market, so I own one and am quite satisfied with it.
They made it so almost everyone uses the Apple TV app for at least some viewing and there you get ads for Apple TV+ shows and their suggestions include shows that require a subscription to a service you may not already have. Or the suggestion will sometimes require a rental or purchase through the iTunes Store. These are ads.
I have no idea what you mean by "they made it so almost everyone uses the Apple TV app." You mean, they made an app that many people like, and that app has ads in it (but not the OS)? That doesn't mean the OS has ads.
Personally, I never use the Apple TV app: I use Netflix, Crunchyroll, HBO Max, and the Criterion Collection apps. And I never see what I would consider to be ads in the OS, and I never see content previews for apps I don't use.
You are free to never open apps that have ads in them on the Apple TV.
(If you mean: installed apps are allowed to show content previews when you hover on them in the UI — I think that's pretty different from an ad, and it's a feature I personally like, since it means I can easily resume a show I was previously watching without even having to open the app-specific UI. That's quite different from my perspective than showing ads for services and apps that I've never used, that I can't remove.)