I don't think you understand how it works. They don't care about the physical media. The physical media itself is not important. The actual value is in the license key. The license key allows you to create an account, and then you can start paying a subscription fee. It identifies you as a user. If you want another account, you have to buy another license. You can either do this by buying a digital version of a game, or another physical copy. It doesn't matter either way. The key point is that there's a buy-in for each individual account.
This traditional business model is different from a free client subscription model, where it's free to download a client and create an account, but you need to pay a subscription fee. Another business model is pure free-to-play where both the client and the subscription is free, while microtransactions fund the service.