Facebook won't launch its news subscription service on iOS due to Apple's 30 percent tax
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The main issue (the only one?) is the standard 30 percent tax that Apple takes from any subscription revenue. On the other hand, Facebook wants to give all the money from the news subscription service to publishers.
The new service will be available to Android users in the next few weeks and will allow publishers to use paywalls on “Instant Articles” which will be enabled after users have read 10 of the publisher's articles in a month.
Furthermore, the “freemium” version of the news subscription service will allow publishers to put paywalls around individual articles.
As mentioned earlier, Facebook will not take any money from the publishers, however, Apple's absolutely must have a piece of the pie, and that's a problem.
source: Recode
Things that are NOT allowed: