My conclusion is that I understand to a certain degree how iOS and OS X differ, and how also the additional content is not the same offer. Yet I think it's just a bit weird how 'yes you can use' and 'no you can't use' intertwine in my example activity of importing an iOS ipad Garageband composition, to show up in Garageband OS X, with non-functionalities and non-referentiality (yet activating a free compatability download), while the (free) second-screen supportive Pro Logic iPad app does provide the interface: why can't I work with that instrument on my mac from scratch then? [By the way, I'm a noob in Garageband, please don't blame me if I missed some option or configuration!]
So to balance the difference arguments with some words from an integrality and unity perspective in order to get real I would state that iOS and OS X are both related OSs and if I buy something or have something at one device in the same-name application, reasonable usability logic provided, then it should be available likewise.