As the music industry finds its footing, streaming services are constantly changing their service offerings to please their stakeholders. As they try and please users, they undoubtedly abandon the needs of labels and artists. And vice versa. With royalty costs increasing, Pandora is faced with the challenge of raising revenue. To do so, they are limited free streaming on mobile devices to 40 hours per month.
Not too bad, but when you think about it, this is probably just the tip of the iceberg. Mobile is indeed making the change, but free and unlimited streaming remains possible on desktops thanks to higher ad rates. Read more over at Pitchfork.

But I listen to Pandora on my phone 40hrs/WEEK!