This may be, in the end, the worst mistake that Apple is doing with their platform. What shape would for instance web shops, or the SaaS market, be in, if the only way to run one was to give 30% of your revenue to Netscape.
Netscape could still be alive this way, but web would be way smaller.
If you're a SaaS app provider, you keep 100% of any revenue you generate through your own website. So if a customer buys a subscription from your website and then downloads your free app from iTunes, you keep 100% of that revenue that you sourced.
If a customer downloads your free app from iTunes and then upgrades to your Subscription offering using an In App Purchase, then you have to share 30%. What Apple is asking for is that if the subscription is generated from inside of the app, you give a 30% revenue share on those subscriptions. That's why they disable links from your app to your website... because those revenue opportunities originate from Apple.
I am not a huge fan of this approach, but it's definitely more defensible than requiring a SaaS provider to give 30% of all subscription revenue to Apple. Apple does generate and give app developers a steady source of traffic/app customers, and it's not unreasonable for them to get some consideration for it. That said (and this is totally subjective), 30% does feel high to me.
Netscape could still be alive this way, but web would be way smaller.