Pandora and Spotify may rake in the money, but they never actually created the content. It would be like me starting a business that resells Microsoft software and complaining that it's too difficult to make a profit because I have to pay Microsoft for the content.
I would be much more impressed if they signed their own artists and made a huge profit.
A popular and easy-to-use medium for finding and listening to music is a valid product, and they deserve to be paid for it. Spotify is useful enough to me that I'm happy to pay for it, and I listen to more music than I would without it.
I hope that labels make enough from these services that they allow them wide enough margins to stay in business.
I would be much more impressed if they signed their own artists and made a huge profit.