How so? I'm looking at the Netflix app in the App store right now and it offers in app purchases. I don't have to sign up before using it. I can sign up in the app (I'm assuming based on this screenshot from the App Store - https://imgur.com/a/t6AqwEx). I already have an account. Not going to jump through hoops to try it out.
You need to logout to see the first time user experience.
Those in-app purchases are grandfathered in from older versions of the app so they don't lose those subscribers. They no longer allow new subscriptions through IAP.
Edited: added screenshot