The premise is not that they're getting unpaid work in the form of programming, just unpaid work in general. If they replace 8 hours of reviewing a candidate's open source contributions and/or side projects with 4 hours of random walks through an undergrad CS textbook, then they're using the candidate's unpaid work to cut down on their HR spending. "How many round piano tuners are there in Fizzbuzz" isn't programming, but it is HR.
Do most programmers have significant amounts of open source contributions? I only had a couple of years of life when I had the luxury of enough time for that sort of thing.
