Some one more seasoned would not work for free during a weekday.
See, I can play that game too.
Besides, this was a contrived problem. If you want to say "I refuse to solve any coding problems while interviewing for a software position", then fair enough. Good luck finding one. I think this was a total of perhaps 3 hours of my time, and was good enough to get an onsite? And it was time I -enjoyed-. Unlike most phone screens, timed interviews, etc.