As we noted when the app was approved, Apple appears to be somewhat gunshy, following the FCC inquiry into why it "blocked" Google Voice on the iPhone (and, yes, Apple still insists it didn't actually block the app, but Google says otherwise). Given the scrutiny, Apple probably realized that it was in for some serious political trouble if it blocked an app like Spotify, which would have received a lot of press attention. Oddly, the AdAge article doesn't mention this at all.
Apple has always viewed iTunes as something of a loss leader to help it sell more iPods and iPhones. If someone else can help sell more of the devices, then more power to them. Though, the fear, of course, is that something like Spotify works on other devices too.
But this brings up the final reason: I would bet that the folks at Apple are pretty damn sure that they can outlast and out-innovate Spotify. Spotify hasn't shown much ability to make money, and while it has become a press darling as a music app, I wouldn't be surprised to find out that Apple's quietly been working on its own version of a Spotify-like offering built directly into iTunes. And, given Apple's standard operating procedure, if that's the case, there's a good chance that the Spotify-like iTunes will be even better than Spotify itself.
So, I don't think it's that confusing why Apple approved Spotify (and Rhapsody). I'd argue that the first reason was the biggest driver. Without the FCC investigation, it wouldn't have shocked me if Apple had denied the app if only to buy itself time. But, I would expect that sooner or later, Apple will come out with its own streaming version of iTunes with very strong integration into the iPhone, and suddenly Spotify won't look quite as interesting.
Having used Spotify a bit, I can definitely see how some people think it could potentially replace iTunes completely. It basically acts like an iTunes that has access to millions of songs at no additional cost (and, yes, it's all licensed and legal). The songs are streamed, but you almost never notice it. It really does feel just like iTunes, while also having "Pandora-like" features for creating specialized stations or sharing others' playlists. Unfortunately, it's only available in the UK for now, though the rumor is it will be available in the US before the end of the year. However, where things could get really impressive is with Spotify's mobile app. For a few months, there's been a YouTube video of Spotify Mobile on Android:
The demo highlights the fact that you can sync any playlist for "offline" play, solving the biggest question about weak mobile signals on the go, or how you use it on a plane or somewhere without wireless access. With offline syncing, it's basically everything that an iPod can do -- with access to 6 million songs without having to pay for each individual song. But, of course, Android is still a limited platform. The big fish these days is the iPhone App Store, and Spotify has now submitted an iPhone app for approval, which raises all sorts of questions. With Apple's history of rather arbitrary rejections -- including ones for things Apple has deemed "competitive" -- will it block Spotify as a rather direct competitor to iTunes? That would be very unfortunate, and again demonstrate the risk of a closed platform.
That said, the initial reviews of the iPhone app seem quite strong. Eliot Van Buskirk at Wired loves the syncing feature, and warns that "you'd have to pry it out of my cold, dead iPhone before I'll delete it from my phone." Meanwhile Music Ally points out that Spotify has uploaded a video of the iPhone app as well:
It really does look pretty slick. So now the ball's in Apple's court. I have no idea if Spotify can survive as a business (and I suspect that the royalty rates the music industry wants will make that difficult), but it is great to see more innovation in the space. Now we get to see how Apple feels about that sort of innovation.