They pretty much do when he is on the clock, you can't do your own thing on companies time, they were paying him while he came up with this stuff, They have a legal argument for sure. If Carmack was doing this all in his spare time they wouldn't have a leg to stand on. This is horrible news for Qcon since Carmack keynote would be awkward now.
Another thing is it is somewhat in the same field of technology, Carmack has a huge conflict of interest if he is working for him self behind zenimax back it isn't like he came up with an idea to create a new condom that has nothing to do with his relationship with id. Would have to see his contract with Zenimax, what exactly was he being paid to do at id if he was a "creative" person with out a defined role Zenimax wins.
One could turn your second argument around. I think there is no problem if Carmack used the ideas he gained while working for Zenimax in his immediate next job, because none of his work was Patented and unless he copy pasted code he wrote and that is owned by Zenimax, he should be in the clear, because his work was dedicated to Zenimax and his gained experience later recycled.
I think they can make a stronger argument about Carmack creating next gen condoms out of the blue right after his retirement. Which he had no prior knowledge of which imples he developed these ideas as a Zenimax employee. In this case he has created something novel as a Zenimax employee of which the company has not seen a dime. I think they have a stronger case like this. But I am no legal expert, it just makes more sense to me.
IMO there is a reasonably strong argument already.
Afaik, Carmack tested, profiled, and advised on component selection for the OR prototype as used to promote it on kickstarter etc. (eg see his timing experiments on that Sony HMD that he complained about being able to send data across the Atlantic faster than to the screen).
He implemented OR support into d3 BFG, which if ZM weren't paid for because JC did it in his free time is clearly ZM property. Remember how BFG was used for all the major shows and press show and tells, and is featured prominently on the kickstarter page.
who on earth doesnt bring the experience and knowledge one has aquired by a job into the next one. what does zenimax expect? him to perform a brainwash so that he looses his know-how?
besides, "proprietary technology" when it comes to software is a huge pile of shit to begin with that is bound to create ruckus like this. hence the whole opensource thing exists to begin with.