Yep, after thinking about it for a while, I can't come up with a good way to argue this from Apple's point of view. The only reason the 30% cut for apps makes sense to me is that it justifies the cost of hosting the apps and updates, the payment service, and so on. Whether or not it's too high is another discussion, though I tend to think it's about right based on the iTunes store still just barley turning a profit. The 30% seems to be the appropriate level to also support hosting the free apps.

Anyhow, 30% for subscriptions just doesn't make sense as a "marketing" fee. If the subscription service isn't using Apple's servers to host content that takes up bandwidth, that takes away the major cost. Payment processing fees are much lower, and even adding in a small advertising fee for bring in new users doesn't approach 30%.