Subscriptions have nothing to do with the Kindle app at all and only partially relate to Netflix. The larger conflict with these apps and the new TOS relate to online purchases of content rather than subscriptions.
This response also isn't very useful because it is a lot more important how the TOS are interpreted in practice by the App Store reviewers than what the original philosophy for the rule was.
(Although I appreciate they probably don't use XSLT and I don't mean to demean what they do).
Assuming it is an edge case, then Apple clearly pushed the edge further out to include more apps under the policy rather than interpreting the grey area in the developer's favor. Edge cases set precedent and the one set by Readability may raise reasonable concerns among developers.
"We said you can drive a car, not a blue car."
We created subscriptions for publishing (the following): apps, not SaaS apps.
Whereas the rest of us see it as:
We created subscriptions for [publishing apps] (apps by content publishers), not [SaaS apps].