>>sorenj+(OP)
If the user is already paying reddit directly, it's even less likely they are using a third party client. All the benefits are in the first party app.
And then Reddit would have to audit third party apps to ensure paying users are getting what they pay for. Sounds more onerous than just making third parties pay up.
>>raydev+v1
I can log in with my Spotify premium account to a number of third party clients without expecting them to contain the same thing as the official one, no auditing by Spotify needed.