Because Apple has had a history of rejecting apps it perceived as duplicating core functions of the iPhone (see the Google Voice kerfuffle for a recent example), and since the currently Europe-only Spotify service potentially competes with iTunes, no one was quite sure how Apple would react.
On the other hand, the Spotify service is primarily subscription-based as opposed to iTunes's a la carte per track/per album model. Spotify's iPhone app will be free to download, but will require a £9.99 per month subscription (or similar annual subscription) for continued ad-free listening.
This news probably also bodes well for the Rhapsody music service app also on deck for approval by Apple. If Steve Jobs and co. have seen fit to approve one unlimited streaming app, it's hard to imagine them turning down a highly similar offering.