> So someone can subscribe to a 0.99/month product and use several 19.99/month products?

Yes, a developer can sell the subscriptions for very cheap but then they will probably quickly run out of subscriptions (there is a limited number) and then wish they had sold them for more.

Also, the subscription is not really tied to any product; think of it more as a subscription to free software in general, that can be sold by different resellers (the developers).