Developers love it. Most developers are making way too much money, and they need to offload at least 30% to get down to an affordable tax bracket. Thankfully Apple comes along with a solution: rather than create a new market which might keep a developer's revenue too high as iOS did, the Mac App Store merely replaces the web to provide the same customer base but in an environment where you aren't saddled with keeping all your income.
As pointed out earlier, they aren't keeping that whole 30%. At a minimum, you have transaction fees on each sales. A lot of shareware developers use digitalriver or Kagi to manage their storefront. That's another per transaction cost in addition to upfront costs and ongoing support of your storefront.
Than you have the marketing benefits of the Mac App Store. How much will that be worth? I'd say more than $0.00. Apple also provides the bandwidth for your app downloads and hosts information about your app. They also manage app licensing which can be a huge cost to support. They provide for redownloads eliminating bandwidth and other support costs.
There's also the savings in development time without the need to create an installer, updater, and licensing system. And support time for those features.
All told, not much of that additional 30% is going to the developer. And if he decides to expand his market by using another retailer, that 30% is in his rear view mirror.
Whether in Apple's Safari or in Apple's App Store, it's still the same 5.6% of the global computer user market available to you.
Well, that is simplifying a bit much. All parts of the market are not the same. Mac customers are more likely to spend money than other parts of the market. And then there's the part of the market that doesn't even know your software exists.
When more people know your software exists, and they've already shown that they are willing to spend extra money on a Mac, and they only have to decide to hit "Buy" (rather than decide whether to trust you with their credit card info), you have a much larger market for your software.