Why your partner development space is on a "Business" plan

Once you've created a development space in your partner portal you will have access to all Business features as long as you do not have any collaborators on the space itself. Your Team Members of the Partner Portal will have unrestricted access to those spaces as well.

Section titled Adding collaborators

Once you decide to add your customers or external users to a development space using the Collaborators feature on said space you or they will need to upgrade.

To make it simple for your and your customer to subscribe you can add one temporary user and assign them the permission to manage subscriptions for you so you do not have to subscribe for your customer.