They're not forced to, no, but they are forced to keep the price in the app the same as elsewhere if they choose to offer in-app purchase subscriptions in their apps; to be on the App Store, you're not allowed to offer a discount to the same subscription cheaper outside the App Store. If there is a generally-available discount (say, for a promotion), that discounted price must also be reflected in the in-app purchase cost.
The net effect is that sellers lose 30% of the potential earnings from the in-app purchases compared to getting subscriptions from elsewhere, and they're not allowed to increase in-app purchase prices by 30% to compensate.
Because Apple disallows sellers from having messages or buttons in their apps that take users to the sellers' websites in order to make the subscription, users who don't know better than to use the in-app purchase system wind up making the sellers less money — in theory. I say "in theory" since, after all, the sellers don't have to deal with payment systems, credit card processing, the sorts of hosting costs associated with popular app downloads, and so on.
On the one hand, it's clearly unfair and heavy handed for Apple to disallow those messages enticing users to the websites so sellers can take the full price. On the other hand, with apps moving to free-to-download but requiring subscriptions to function, especially big ones like Netflix and so on, it mightn't be that surprising: a 30% cut of the subscriptions might turn out to be rather reasonable given that Apple's hosting the App Store, addressing and directing complaints, handling payments, and more.
On yet a third hand, companies don't need to provide in-app purchases for subscriptions at all, mandating that users go to their websites. It's up to the sellers to decide what provides users with the least friction, though.