Primal is optimizing for new users so that they have to configure as little as possible for the app to just work. The caching relay set as the ONLY relay the client is reading from fits right into this objective. Users don't have to wonder why they don't see notes on Primal that are visible elsewhere, because chances are the caching relay has pulled them in from other relays. Users don't have to go and make sure they are reading from the relays that their friend who told them about Nostr is writing to for the same reason.

That being the case, I don't think they would ever make the caching relay an optional service that is only available to premium subscribers. It is too fundamental to their user-friendly UX.