As others have pointed out, saying that "Apple has a monopoly on selling iOS software" is like saying that "Sony has a monopoly on selling PlayStation software". It's very unclear that there's anything monopolistic (in the legal sense) going on here. I personally don't love the direction that this kind of thing is moving, but that doesn't automatically make it illegal.
There is no doubt they are overcharging by huge factor.
I don't really agree. Sure, 30% seems like a big number - but then you need to consider what you get for that 30%:
* QA resources. This isn't just a rubber-stamp; they really do pay attention to your software and how the software might affect users. No, it's not a complete replacement for in-house QA, but it does mean that a modicum of quality is required before an app can be sold on the app store. While you as a developer might say "we have our own QA resources, trust us!", many users on the other hand are saying "we don't trust a lot of the developers" and rightly so.
* A store-front. Larger companies can probably tackle this themselves without sweating, but for a smaller company or one-man operation, this can be a big deal.
* Card processing, transaction fees, etc. Yeah, this is only a few percent, plus a bunch of extra work that you don't need to worry about.
* Massive audience. Very few companies can achieve the kind of audience that the App Store can give. On the flip-side, a large audience doesn't guarantee that anybody will buy *your* software, so it's not a magic bullet.
* Discounts. I have often walked into my local supermarket and walked out with a "30% off" prepaid card for App Store purchases. For any apps I buy with that card, Apple is getting around 0%. Sure, not all users are taking advantage of this, and sure, it's perhaps benefiting Apple in a not-directly-financial manner, but 0% is still 0%. More common perhaps are the 20% discounts, so Apple is getting more like 10% there (and remember that all of the above costs and fees are still coming out of this 10%).
In short, if you're a big player, you may be able to slim your costs below 30%, build your own store, run your own sales, bring your own customers, build your own brand trust, and so on - but tbh not that many developers fall into that kind of category. In the meantime, there are many advantages to consumers to being able to trust Apple's known-quantity storefront.
Again, I'm not saying that I don't see the downsides of this model - there definitely are many - but claiming that apple is simply biting off a big chunk of the developer's revenues is a vast, vast oversimplification.