The reason why Apple charges 30%—and is justifiable in doing so—is because the App store itself is a marketing platform.
If a user discovers the Netflix app, signs up, they are essentially discovering Netflix through the App store.
If a user discovers Netflix elsewhere, they can be pointed to download the app for extended utility.
In the latter case, it doesn't violate Apple's 30% revenue cut policy. But if they want to target people who discover Netflix from the App store platform, then they would have to pay.
As another commenter pointed out, the "discovery surcharge" makes sense for a one-time purchase. It makes less sense for an ongoing subscription.
Also, does anyone actually discover Netflix through the App Store? Or do they know about Netflix, go to sign up, and the first device they sign up on happens to be running iOS?
30% is the expected amount for an entire marketing budget. Back when the app store wasn't so overrun, it may have made some sense. (Remember, Jobs justified it when facing initial criticism by saying that most people were going to use mobile web sites anyway so the app stuff isn't really a big deal.)
Now, you have to give the cut and do marketing. It's usurious, and we need to stick to other channels.
If a user discovers the Netflix app, signs up, they are essentially discovering Netflix through the App store.
If a user discovers Netflix elsewhere, they can be pointed to download the app for extended utility.
In the latter case, it doesn't violate Apple's 30% revenue cut policy. But if they want to target people who discover Netflix from the App store platform, then they would have to pay.