Both Spotify and Elon Musk have previously spoken out against Apple’s App Store guidelines in the past. In a new exchange on Twitter this weekend, Musk and Spotify CEO Daniel Ek again questioned Apple’s guidelines, with Ek referring to them as “absurd” and Musk saying they present a “serious scaling problem.”

The conversation began when Twitter launched support for all users to offer “subscriptions” on the platform last week. Musk followed up on the announcement, after questioning from users, to explain why subscriptions take longer to appear on iPhone than they do on the web.

“Note, it takes a few days longer for subscriptions to go active on iPhone vs. web, as all subscriptions currently have to be approved by Apple,” Musk explained.

This is in reference to App Store guidelines around in-app subscriptions for digital content, which Twitter must abide by since these new subscriptions are available via the iPhone app.

Spotify CEO Daniel Ek then quote-tweeted Musk’s post and again criticized Apple’s guidelines:

Ek: “This is absurd… How would this scale with every creator on every platform on the internet? And what about if a platform thought the right fee was 0% or 10% instead of Apple’s 30%?”

Musk: “This is becoming a serious scaling challenge.

Spotify has been a vocal opponent of App Store guidelines in the past. The company has even filed antitrust complaints against Apple in the EU, arguing that Apple can offer Apple Music subscriptions within the app without any penalty, while Spotify would have to give Apple 30% (or 15% from year two) of its subscription revenue if it did the same.

For his part, Musk has previously said that Apple’s App Store fees are essentially like having a 30% tax on using the internet.

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