The bulk of their revenue comes from customers who subscribe on a per-month basis, while they pay out royalties on a per-song-played basis. This outage is reducing the amount they have to pay, and if the outage-elasticity-of-demand is low enough they could (hypothetically) come out ahead!
I believe this is inaccurate. They pay out royalties on a share-of-all-plays basis, don't they?[0] So an outage wouldn't reduce the payout amount, it would just slightly alter the balance of payments for individual rightsholders.
[0]http://www.spotifyartists.com/spotify-explained/#royalties-i...: "That 70% is split amongst the rights holders in accordance with the popularity of their music on the service. The label or publisher then divides these royalties and accounts to each artist depending on their individual deals... Spotify does not calculate royalties based upon a fixed “per play” rate."