Because it’s part of the show? What is it advertising? Usually there are some amount of credits. Certainly not ads.
Maybe I’m misunderstanding the post. I was under the impression that they are talking about intros/ads to other shows on the same service.
Example: I start a brand new show and I’m immediately hit with an ad for another show on the same service, before I can watch the show I actually selected.
Are you taking about previews? Yes, those are ads. The intro to a show that is the same every episode, those aren’t ads, but are skipable.