Licensing, definitely - PLUS, it might also have been an area of territories, as there's different deals for Europe vs North America, etc.
They likely licensed Sab for broadcast, but when it comes to *selling* product, that's a no-no. A rental is a temporary sale. Now that doesn't mean there isn't going to be Sab music in other rented movies or shows, it simply means that this particular production did not have permission to use it for the sale of *that* show. And even that can have different implications - It might have been a limited-time deal, or a deal for broadcast-only, or perhaps the owners of the publishing pulled permission (which is a standard clause in these types of contracts). ((OR, upped the price if they saw there were big bucks changing hands, wink))
I hate lawyers,