How do you catch up with the biggest music streaming service? Well, not approving app updates is one tactic, and Spotify says Apple is doing just that. The streaming service sent a letter to Apple's legal counsel this week claiming that the company is rejecting an update to Spotify's iOS app and it's "causing grave harm" to users by doing so. The letter explains that Apple won't approve the new version because Spotify doesn't use the company's billing method for in-app purchases and subscription services. Apple announced the changes to app subscriptions in iTunes just before this month's WWDC.