Reader apps are a defined category of apps (in the App Store guidelines) that provide access to digital analogues of traditional media rather than providing more abstract services.
The two advantages that reader apps have here IMHO are:
- There are a lot of cases where apps which provide services need to provide an in-app purchasing option going to Apple (you can take money separately, but the user needs to be able to get functionality from the app without leaving the app). Reader apps have a bit more flexibility for no in-app purchasing.
- Reader apps are now able to call out to out-of-app purchasing in more cases, such as in email correspondence. This is likely to continue to expand from regulatory pressure, since Apple is growing their competition in media services.
Generally Apple's model is that apps are partners with an agreement that Apple charges for services as well as for the corresponding marketing and customer acquisition. Things like linking to an out-of-app experience in order to avoid paying for customer acquisition, or taking money directly in-app without giving apple their cut is the quickest way for them to decide not to work with you anymore.
Reader apps were a carry-over from the iPod days where you could load arbitrary third party music onto Apple's devices. As Google has learned, you get far more regulatory scrutiny when you _tighten_ rules over time as you get more marketing power.
Apple has really only added new services or loosened rules, although they have had rules which they were more lax on enforcing at times in the past. That said, they have added new media services which will may count as them trying to leverage their platform to get into new markets, and hence they may have more 'reader' style concessions (for instance, for Peloton-style fitness services)