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.