They put up a post explaining that raising royalty costs are part of the reason. Pandora also states that the limit of 40 hours per month will only affect about 4% of its total monthly active users and is only for mobile devices. The average listener is only pulling in 20 hours of streaming per month. With per-track royalty rates jumping more than 25% over the last three years, it isn’t hard to see why Pandora needs to make a change, even if it is against their original mission.
There is an inexpensive way out of the limit if you are part of the estimated 4% though. Your free service will still be available for unlimited listening on a laptop or PC, you can pay $0.99 for unlimited listening for the rest of the month or subscribe to the service. The plans are $3.99 a month or $36 a year.