It's that the interview process proposed by the G[...]P may force the candidate to do things that may be illegal to pass the interview. The candidate is of course responsible for their own choices, but the point is that as an interviewer, if you force your candidates to do this, you might short list those who have a tendency to violate contract terms.
This might work out for some, but I'd say it's a fair point to bring up.
An asshole filter https://siderea.dreamwidth.org/1209794.html (maybe you've read already?)
I'm an employer right now in my small company. Don't see any reason to screw up my employees, on the contrary, I see it's much better to work in a friendly environment. There's no such "unimportant but necessary to the business" things.
What's your source?
"unimportant but necessary to the business"
What examples?
Also, if they don't work there anymore, they should've deleted all the code (if they had it) and surrendered/cleared up all the laptops.
But I like your idea, anyway I give big kudos to candidates with their own pet projects.