You're sort of moving the goal post now. First it was they're getting exposure for free. Now it's the $99 doesn't cover Facebook's downloads. If Apple needed more to cover the overhead on serv... no, no, no. I'm not going down that rabbit hole.
It makes no sense on too many levels. Spotify is most likely not as put upon as their message makes them out to be. This is a perfect opportunity for them to highlight the unfairness of the 30% rip in perpetuity. This issue goes beyond Spotify and affects any app with subscription services. If Apple took 30% from the initial set up only, I'm sure that would be more acceptable. If that was the case, Apps could charge the same via their website or the App Store and customers benefit. In the end, that should be the goal; benefit the customer. I know it's a bit Pollyanna, but still. It would crass to say Apple doesn't need the money, but it would be just as crass to try to make excuses like paying for servers when Apple is stacking up profit, not revenue, but profit like no other company in the world. Apple is even less sympathetic when you add the fact that they 1. make the subscriptions go through their processing, 2. won't allow devs to let customers save by linking to their websites, and 3. won't let the devs tell the customer to go to the website to sign up. They don't exactly present the picture of the customer focused organization.