Spotify is pay-to-play and the royalties model is designed around that. Artists/Labels get a a very small slice of money from every play. As a Spotify consumer you do not pay for the right to play the track offline - if you did, the Artist/Label would get a bigger slice. If you want to play the tracks offline, just buy the non-DRM MP3 from Amazon or some other place.
For people saying the Labels take all the money anyway, there are many independent artists on Spotify who deserve your money if you play their work.
Most of us here are hard working professionals in our fields - how would we feel if someone said at the end of each week, "Thanks for what you did, it's really useful, but I've decided to not pay you, but I'm going to keep your output anyway and use it forever." We wouldn't stand for it, and nor should music artists have to.