Taylor Swift's 1989 has sold almost 9 million copies, worldwide, but you won't be able to find it on Apple's soon-to-be music streaming service.

On June 30, Apple will launch Apple Music, a subscription-based streaming service not unlike Spotify. In a post headlined To Apple, Love Taylor, the singer explained why she was withholding her latest album from Apple Music.

Although the service will have a monthly fee of $9.99 a month, Apple is offering a free trial membership for three months, which is what rankles Swift. "I'm not sure you know that Apple Music will not be paying writers, producers, or artists for those three months," she writes. "I find it to be shocking, disappointing, and completely unlike this historically progressive and generous company."

Although Swift is one of the best-selling contemporary artists, she says it's not about the money but the principle. She goes on: "These are the echoed sentiments of every artist, writer and producer in my social circles who are afraid to speak up publicly because we admire and respect Apple so much. We simply do not respect this particular call."

"Three months is a long time to go unpaid, and it is unfair to ask anyone to work for nothing," she writes. Swift has long been an advocate of proper compensation for musicians. In an op-ed for the Wall Street Journal, she wrote, "Valuable things should be paid for. It's my opinion that music should not be free." She later had her catalog pulled from Spotify, unhappy with their business model. "They just need to be a better partner," said the president of her record label.

Unlike Spotify, Apple Music will not provide any free, unlimited streaming outside of a trial membership (Spotify's free subscription comes with ads). Swift applauds Apple for "working towards a goal of paid streaming," but says they have more than enough capital to provide compensation to artists during a the three month trial period.

"We don't ask you for free iPhones," she closes the letter. "Please don't ask us to provide you with our music for no compensation."