I should maybe have gone into more detail on that example. If BigCompany or even AcceleratorCompany asks for product-relevant work, I'm not going to assume they're having random people off the street write production code.
What I've seen is something like "this is a startup in market X, whose product doesn't do Y but could, and they're asking candidates to write code that does Y, and sign over all rights to everything provided during the interview process".
Optimistically, it means that if whoever they hire will be told "hey, first assignment is Y, you've already made progress!" Pessimistically, it goes in the same category as "Whartonite seeks codemonkey" and "sign this NDA to interview" - it's a matter of ego and ambition exceeding skill.