For example, even though a lot of people say they'd rather learn from written articles, you can sell a bunch of videos in a set for $249 in a way that the equivalent set of articles would not sell.
Likewise, there have been experiments with selling free Web content on the Kindle for a few bucks.. and it sells like hotcakes despite being free on the Web. These are interesting times and it's worth going with what people are doing rather than what people are saying they want. (Though while always experimenting and ensuring you don't ignore your audience.)
Plus the idea that you can do something this seamless and elegant on the web is horseshit. You can't subscribe to any web magazines in under ten seconds. (and if you could, odds are that they'd fill your inbox with spam.)
The only reason I can imagine, and mind you it is a BIG reason, to make The Magazine an app is the business model.
I read the web on my laptop, differently than I consume content iPad. When I read "The Magazine" it launched the App, and it took over 100% of my iPad experience, and I spent the next 20 minutes reading through the four (excellent) articles, completely free of distraction.
I'm willing to pay $2 for 8, carefully curated, selected, edited, and written articles from Marco Arment, presented to me on my iPad.
Those 4 articles, on Life/Love/Technology, on Volatiles vs Stables, and the impact on how to manage engineering companies, on the concept of "Voice" and the linkBlob/Fireball format, and the schism in Geeks who Love Sports, and those who don't and really, what is the central essence of being a Geek - are timeless, and I'm confident I'll re-read in months to come.
Finally - I love the idea of finding an outlet to sponsor those who contribute so much of the great content that I consume, in this structured way. I hope "The Magazine" is very successful, and provides an income for those who write such great material.
I think people should view this as iOS first rather than iOS only. I think it's a pragmatic decision rather than some sort of "iOS or death" principal.