They are asking the interviewee to complete the same task as done by an existing staff member, and compare the results, how is that "free labour"? I'm not sure

They are asking the interviewee to complete the same task as done by an existing staff member, and compare the results, how is that "free labour"? I'm not sure where I stand on this sort of thing to be honest, my only experience of anything similar was a pre-interview task to implement a simple 10 entry LRU cache, and after getting the job I was told the only reason it was there was to weed out time wasters with next to zero skill being sent by the recruitment company.