I think you're grossly underestimating how much 30% of revenues means. Let's look at how this could actually affect you. Lets forget for a second that Amazon is even part of this issue. Let's stick to straight software development, what HN knows best. Let's look at the profit margins of some of the best software companies:
http://www.google.com/finance?catid=us-54399928&sort=ANI...Look at what percentage of revenue that net income is, and count how many are above 30%. Now, isn't it a bit more difficult to make money on a product if your 30% margin is gone? Sure, Apple deserves some money for payment processing and distribution, maybe something for acquiring a new customer but certainly not 30% of all revenue.
Yes, you may say you can just raise price, right? Well, to compensate you would have to raise price over 40%. Meanwhile, the customers have already demonstrated the prices they are willing to bear, so that doesn't quite work.
So now, if you've started a business of developing iOS apps, you're severely hampered by this 30% revenue cut you're giving to Apple. The small guys will probably deal with it because they won't know better, but think of it this way: Apple has just taxed 30% of the value you have created for your users. That's 30% of your income.
You can argue this is a retail model and they should get a margin the same way you do if you sell something through Best Buy, but it becomes anticompetitive when they've blocked off any option you have to sell directly to a customer or through any other channel. The companies on that list do very little business through retail, and they would suffer if they had to give up 30%. Moreover, Apple's own margins would take a hit if they had to sell everything through a retail partner.
(Also don't get why it matters that Rhapsody, Netflix, Amazon don't create content. Apple doesn't create most of the stuff that goes into an iPhone, but that doesn't mean they don't add value and shouldn't be able to make money)