I understand what you're saying, and I'm not going to adapt the analogy to suit my view of the situation because that invariably becomes kind of silly.
I think that as long as Apple allows publishers to make content viewable inside the app store purchasable outside the app store only, this policy is fair and as you've described it.
If Apple requires publishers to make content viewable in the app store purchasable in the app store, the publisher is forced to choose between losing 30% of revenue on a chunk of their transactions or having a vastly inferior product for entirely artificial reasons. Apple would essentially be holding the publisher's product quality ransom.
I'm also not a fan of this because I think that people who have costs forced on them for a specific subset of their transactions should be able to transparently pass on those costs to the customers who make those transactions, rather than spreading it out across all customers. I think that publishers should be able to charge 43% more for purchases and state in the app that it costs more because Apple is taking a 30% cut. I think that retailers should be able to charge 1-2% more to customers who use Visa and MasterCard than to those who use cash, and a couple percent more for Discover or AmEx. Instead, the cost is concealed and the users who don't have CCs with excellent rewards programs are paying for the rewards programs of those who do. With this policy, non-iDevice-users will pay a premium to allow iDevice users to buy the content more easily, and that doesn't strike me as particularly fair.