Hulu only allows in app subscriptions for the streaming video on demand level. They don’t allow in app purchases of the much more expensive Hulu Live offering.

As far as I know, you can’t subscribe to any of the cable alternative streaming services via in app purchases (Hulu Live, YouTube Live, Sling or DirecTV).

New subscribers to Netflix haven’t been able to sign up through the App Store for ages.

It’s very much a business decision whether they allow in app purchases.