> What is the employer putting up? Generally nothing.

Unless you've been in the shoes of a hiring manager, I'm unsure as to where this idea is coming from?

Having been a hiring manager, it is a horrible experience. Unless it's extremely naive, homework sets generally take time to come up with and formalize, and certainly take time to review the results for.

I suppose I can understand conflating the motivations of an interviewer with their employer, but it's not generally true to say that interviewers are just trying to exploit those they interview.