only needs to be available as an in app purchase if the app itself has a way of enabling you to purchase same content from outside the app (for example, a "subscribe now" button that links directly to a purchase page in Safari). I'm not sure where just a "you can subscribe from our website", sans link, would fall, but I'd hope that would be ok.
If it's available for purchase/download elsewhere it also needs to be available in-app for the 30% fee. It can't be priced higher in-app. That is the dilemma for the retailers.
Here are the choices:
1) Pull out of IOS completely
2) Lose money on all in-app purchases and hope that you make enough in direct sales to stay in business
3) Raise prices everywhere so the 30% fee doesn't hurt so much
Option #2 is just not realistic. It could be a very tough call between #1 and #3. The IOS store probably has enough critical mass that a business could lose substantial sales by ignoring it. The bigger risk is allowing an opening for a new entrant to come in, build a business in an untapped market and eventually be a tough competitor. Option #3 basically screws all your customers- even if they've never even heard of the IOS stores- so Apple can line their pockets.
It's a really crummy policy. It's either going to hurt businesses, who stand to lose sales or even go under, or it's going to hurt customers who will have to pay higher prices as a result.
(4) negotiate a deal where they get more then 30%.
(5) negotiate to remove DRM and operate as a Web App.
(6) change business models to subscription.
(7) license the app part of the business model to another company not involved with your store. That company can then pursue anti-trust claims against Apple if disallowed since they have no ability to offer the items for sale.
(8) add more value with your product i.e. get a better business model then "I'm gonna get rich being an additional middleman of ebooks"