But your point still stands. They are competing with piracy, and doing a good job too.
And InformationIsBeautiful has an infographic showing how many plays a solo artist would need on each digital service to earn the US minimum wage: http://www.informationisbeautiful.net/2010/how-much-do-music...
These sites might be solving the service problem, but they're not earning much money for the artists.
Spotify may well be providing significant revenue to the labels, but as Derek Webb pointed out, it's a raw deal that devalues music in general with little upside to anyone in the industry (http://derekwebb.tumblr.com/post/13503899950/giving-it-away-...).
Now sure, maybe labels have been overpricing music with $20 CDs and they're ripe for disruption. But at the same time, why should artists be keen on trading in their overpaid RIAA-type management for a handful of small tech startups with no credible upside (how viral would you have to go to earn a million dollars?). The fact that Spotify is consumer-friendly means very little to a starving artist. If I were a musician I certainly wouldn't be buying into that system. Instead I'd be tolerating piracy and doing what I could to differentiate myself in the high-value channels (ie. people buying music).
If I were a musician in it for the money, I'd put my music out on every available channel (even free ones), then make money on tours and exclusive merchandise.
Way back before piracy and good streaming services, the average person could not easily discover new music. Now they get exposed to all sorts of stuff through these channels, and there is very little barrier to listen to something. This then turns into people buying concert tickets etc.
As easy as it is to hate the big bad RIAA, it's a red herring. Musicians are under no obligation to support any particular business model. And contrary to what you say, if every musician puts their music on Spotify, then Spotify becomes the obvious consumer choice, and that sets a maximum value that music is worth, and that value is orders of magnitude less than it can be worth under the iTunes model. Musicians don't have to put their music into Spotify though, and as long as significant numbers of them don't then the higher value market for music can still exist. Piracy is definitely inevitable, and I believe it's a waste of time to try to fight it at the individual scale, but there is no reason artists have to bend over and accept a fraction-of-a-penny pay-per-stream model as inevitable.
EDIT: I'm not disagreeing with the giving it away for free premise, but the "streaming won't make me any money" part.