Other software you run is billed relative to your MSU tier. So, if you run z/OS then your cost will be higher if your machine has more MSUs. A weird quirk of this is that there is thing called "IFLs" (Integrated Facility for Linux) which, when I when I first heard of them, I thought was a separate processor designed for for linux. However, it is not. It is actually the same as the regular processors that run z/OS etc, the difference is that is is licensed exclusively for running Linux (or like z/VM to run linux counts too). The reason for this is to enable shops that want to run linux and needed extra horsepower to do so, but didn't want their z/OS bills to go up because they purchased more MSUs. So, despite buying more of the processor capacity within the mainframe, it doesn't count towards the "MSU" number that impacts the cost of various software because you are using with one type of software vs another type of software.
All that being said, in my opinion, it needs to come with several features:
1. no subscriptions for something that is just a one time unlock 2. It needs to be legal and protected for customers to figure out how to unlock it on their own without purchasing the unlock.
I haven't thought enough to have a strong opinion on the exact situation you describe (where it is present and an unlock isn't available at any fee), other than to say I'd still argue strongly that customers figuring out how to unlock it on their own should be legally protected.