In what bizarro world is an email reader not allowed to use rules that apply to "reader apps"? Especially when things like video and music services, that involve no reading, can.
This doesn't mean the Guidelines are foolproof and always consistently applied.
If you're just trying to read the Guidelines as legalese then it seems to me like there's some serious ambiguity with what exactly the "Reader" section allows.
It seems like Basecamp gets a pass based on the "access to professional databases" exemption. But at what point does a service that caters to consumers AND businesses cross over that line?
The App Store Review Guidelines have been a nightmare to parse through and the Review process an inconsistent mess for many years now. IAP really took it to a whole new level.
Smaller subscription video or music apps could use the same section in the App Review Guidelines for "Reader" apps to have an app without in-app purchases that you buy outside the App Store.