So, there's a whole host of problems with this line of reasoning.
What we do as engineers, especially in software development, shouldn't be treated with the same reasoning as you'd seen in an assembly line.
First, note that Carmack was a cofounder of id; a good chunk of his precedent and history was doing exactly this sort of research work. Does Zenimax want to go back and claim ownership of his contributions to graphics cards manufacturers for his help with their drivers and APIs?
Second, note that at a high level, it simply doesn't make sense to have an engineer not working on R&D in fields outside of what the current core line of business is. Especially in a AAA studio, a lot of that work is either art or scripting or quashing bugs--a workload which is honestly a total waste of senior engineer talent. They enjoyed the fruits of this labor; consider the extra sales of Doom 3 BFG whose improvements were driven partially by this work with Oculus.
Third, Zenimax wasn't and isn't in the business of making hardware, and only now seemed to give a shit following the Facebook acquisition. This is so transparently a cash-grab that I'm kind of surprised you are being generous at all on this.
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Look, I don't know about you, but I'd rather not live in a world where my employer effectively owns everything I do while drawing a salary there, and can make a claim on everything I work on hence, regardless of whether it makes any logical sense or not.
Further, I don't think it's good that you can spend twenty years building a business (industry, more honestly) and then have some asshole suits screw you out of what should by right be a fun working semi-retirement. It's like spitting in the face of engineering's American Dream.