Under that theory of law, every platform company in the world is subject to antitrust suits as the "monopoly provider of access to that platform". Twitter could be sued. 37signals could be sued.
Fortunately, this simply isn't how antitrust law works.
What theory of law? Notice the words 'weird' and 'starting to live', I am saying that this is new territory and as such the law hasn't even been tested yet. It may even result in new legislation.
There is a huge gap between the monopolistic marketplaces that Apple and MS have setup and service providers like Twitter or 37signals who give api keys to anyone who asks.
And why fortunately? Do you like having your freedoms curtailed? Do you like being vastly overcharged for things you know would be a lot less if competition was allowed?
Microsoft's monopoly was only on Windows, which just happened to be the only viable option for most PC OEMs and browser developers.
If owning 99.4% of the mobile app sales market (Gartner figure for 2009) and imposing restrictions which hinder the development of cross-platform apps doesn't deserve antitrust scrutiny, I'm really not sure what does.
No, it was the only option for most PC OEMs, because Microsoft adopted anticompetitive licensing practices that explicitly punished OEMs financially for distributing anything but Windows with prominently-placed IE.
Apple cannot control 99.4% of the mobile app market, regardless of what their current revenue share is, because they still control less than 30% of the market for app platforms. If Apple attempted to abuse their position in the market to the detriment of customers, customers would switch to other phones, which is easy because there are multiple vendors with approximately the same or greater market penetration.
There is just no way to get around the fact that Apple does not control the mobile app market (yet). Coming up with the market model that maximizes revenue and one random Gartner stat does not make them a monopoly. Read the document I posted earlier; it's written for laypeople.
Think of it this way: imagine Apple invented 3D animated wallpaper technology, and allowed people to create and sell wallpapers in a special wallpaper store. A year later, Samsung releases a phone that also has a 3D animated wallpaper store. By your logic, Apple would have nearly 100% share of the 3D animated wallpaper market, and would be subject to antitrust regulation.
OEMs wanting to offer a non-IE default browser had the unattractive options of switching Linux or incurring the financial penalties; iPhone developers barred from using "cross platform" tools had the unattractive option of switching to platforms that have shown relatively little potential to generate revenue or higher development costs.
Because of their store cornering the market for paid apps, Apple is in the enviable position of being potentially able to reduce development on their competitors' platforms, thereby making customers less likely to switch as well as worse off overall. Irrespective of the letter of US antitrust law, that is something I feel _ought_ to be kept under scrutiny.
Fortunately, this simply isn't how antitrust law works.