Reader apps — ones whose core function is consuming content bought elsewhere, like an e-book or video subscription app — can allow account access and content viewing without forcing In-App Purchase, but still can't include a direct purchase or upgrade flow outside IAP.
A common misstep is a Reader app going a step further and including an actual "buy now" or "upgrade" button that links externally, which goes beyond what the Reader exception allows.
"Your app includes a button or external link for purchasing content, which goes beyond what is permitted for Reader apps under guideline 3.1.3(a)."