I am guessing a bit here, but maybe someone who knows better can jump in on this.
Anyway, maybe that's why they attach it to the runtime and try to call it a "runtime fee." Since you, An Esteemed Game Developer, are shipping a game that includes their runtime, you more or less have to have some kind of active relationship with them, or else you'd be shipping their runtime illegally. And since that relationship is active and ongoing, they're able to change the parameters of it because if you don't agree, then you more or less lose your ability to ship your game mixed with their code? Like I said, kinda guessing at some of this.
Yeah, it's complex. There's some ambiguous language in the current-ish versions of the TOS that talks about how you have to delete all copies of the "Unity Software" when ending a subscription - the intent is that you delete the editor if you're not paying for it, but there's enough legal weasel wording that you could see them claiming that this also impacts the Unity Runtime and Unity Editor together if they wanted to be absolute assholes and burn every bridge they had. It would be extreme bad faith, but hey, we're talking about JR here.
HOWEVER
Some of us shipped games before Unity was in a subscription model in 2016, and a lot of console games even past that point shipped on the Unity Runtime without a subscription thanks to some real ridiculous quirks with how protective they were with shipping console code and who paid for what. There's zero room to revoke or change terms on these cases. There's also the Unity statement about "if you're on LTS, your TOS is also bound to that time period" which is pretty rough in terms of intent for them.
For us with 2014 and 2016 titles, it puts us firmly in "bless your heart, good luck with that!" territory. We switched in 2017 to Unreal partially because of the sad state of Unity 5, but mainly because we needed to make sure we had control of our destiny - Unity's lack of response to engine issues when we were trying to get into cert nearly killed our studio, and I refused to let a middleware vendor have complete control over our ability to patch and ship ever again. It's super heartbreaking to see our fears were right in a way that will impact hundreds/thousands of developers.

